Trilobites occur as fossils only within the rocks of palaeozoic ages. They suddenly appeared in the lower cambrian and reached their maximum in cambrian and ordovician period. In silurian, they were still abundant but became less important in devonian. With the end of the palaeozoic era, they became extinct. Geological age of some important species:

(i) Olenellus:

Low-cambrian.

(ii) Paradoxides:

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Mid-cambrian.

(iii) Olenus:

Up-cambrian.

(iv) Calymene:

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Ordovician.

(v) Illaenus:

Ordovician.

(vi) Phacops:

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Silurian.

(vii) Redlichia:

Mid-cambrian.

(viii) Phillipsia:

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Carboniferous to permian.

(ix) Asaphus:

Ordovician.

In view of their wide geographical distribution and rather a limited range in geological time, trilobites are regarded as excellent index-fossils.

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Echinoids:

The echinoderms, belonging to the class Echinoidea, are also known as sea-urchins. They always possess a compact and rigid, more or less a globular, heart shaped or discoidal body covered with spines. The shell or test is covered by a layer of ectoderm and consists of numerous calcareous plates which constitute the exoskeleton. Some of the echinoids show radial symmetry and others show bilateral symmetry.

The lower side of the test is known as the oral or ventral side. The upper side of the test is generally convex in shape and is termed as the dorsal or aboral side.

The mouth is situated on the oral side, either in a central position or in front of the centre. The anus is either at the summit of the test or posterior to it. The mouth of the animal is encircled by a number of plates which collectively constitute what is known as the peristome. In the same way the anus is surrounded by a number of plates, which together constitute what is called the periproct.

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Sometimes both the mouth and anus are placed at opposite poles of the test and in some other echinoids the anus is always and the mouth is often excentric. Accordingly, the echinoids are divided into two orders like

(i) Regularia

(ii) Irregularia.

The test or shell of an echinoid is made up of three parts:

(a) Apical system or apical disc,

(b) Corona,

(c) Peristome.

Apical disc:

Around the anus there is a flat series of ten plates called the apical-disc. Of the ten plates, five are known as occular plates and the rest are called the genital plates. The genital plates are larger than the occular plates and they lie in contact with the periproct and form the inner part of the ring. One of the genital plates is perforated by a great number, of smaller pores, through which water filters into the water-vessels: this plate is known as the Medreporic plate. All the genital plates are interradial and are termed as basals.

The occular plates lie in alternation with the genital plates and constitute the outer part of the ring. They are termed radials. When the occulars separate the genitals and touch the periproct, they are said to be insert and when they do not touch the periproct they are exsert.

(b) Corona:

It is the main part of the test made up of twenty columns of plates; each column extending from the apical-disc to the peristome. The plates are of two kinds:

1. Ambulacral plates.

2. Inter-ambulacral plates.

There are five double columns of ambulacrals separated by five double columns of inter ambulacrals; each double column is termed an area. The ambulacral plates start from the occular plates and the inter-ambulacrals from the genital plates.

In each area, the plates alternate on either side and since their inner ends are angular, the line between the two rows is zig­zag, while those between two adjacent areas are straight.

The ambulacral plates are smaller and more numerous than the interambulacral and are perforated by pores for the passage of tube-feet to the exterior. The tube-feet are called podia, which are the organs of locomotion.

In the umbulacral area, the more aboral plates are called simple-primaries which have one pore-pair in each plate. Lower- down, compound plates appear. Each pair of pore is surrounded by an oval raised rim, known as peripordium. Compound plates also consists of various types of plates known as demiplates, occluded plates, included plates etc.

Echinoids are often provided with rounded elevations known as granules or tubercles. They are of various sizes. The larger ones are called pirmary and the smaller ones are called the secondary.

Parts of a tubercle:

(i) At the top of the elevation, there is a hemispherical piece with a perforation (at the top) known as mamelon.

(ii) The mamelon rests on the boss. The base of the boss is generally surrounded by a smooth excavated groove known as areola or scrobicule, to which the muscles of the spines are attached.

The spines consist of the following parts:

(i) Acetabulum:

It is that part of the spine which fits to the rnamelon.

(ii) Collar:

Next to the acetabulum is the head of the spine above which lies a ring called ‘collar’.

(iii) Beyond the collar and forming the greater part of the spine is the shaft or stem, which may be smooth or ornamented with ridges or rows of spiny processes.

Sometimes very small tubercles occur in the form of bands. Such bends of minute tubercles form what are known as fasciole.

(c) Peristome:

It lies on the ventral side of the test and the mouth of the animal occurs at the centre of the peristome. The peristome is made up of plates which are loose and are either iso­lated or occur in pair. The position of mouth in the peristome also varies in different forms. Peristome is usually a memberane that surrounds the mouth.