or SOUTnERN INDIA. 
7 
Eor reasons -wliicli have already often been alluded to, the study of the 
Echinoids is of very great importance. In tracing the successive forms as they 
followed each other in course of time, there is an unmistukeahle gradual change 
observable from the si^hcroid to the hemispheroid and bilateral types. Mauy of 
the genera and species are most characteristic for certain formations and even for 
single beds, setting aside the certainty with which identifications of species can 
often he carried out. With the termination of each epoch, hut particularly the 
cretaceous, many very important intermediate links have disappeared, and the study 
of these fossil forms is, particularly in this class, absolutely indispensable for pur- 
poses of a correct aud natural classification. Hence the importance of the Echi- 
noids for the geologist as well as for the zoologist. 
The knowledge of the fossil species has, especially within the last few years, 
been very considerably enlarged through the labours of Desor, Cotteau, Laube^ 
and a few others ; but that of the recent forms has scarcely made equal progress, 
though some important observations were made by E. v. Martens, Sars, and 
others. It is ouly to bo regretted that our anatomical and morphological know- 
ledge of the entire order is as yet very small. 
A short abstract of the principal sub-orders and families may he found useful, 
more details will he added further on. The two principal divisions of the order are, 
as already noticed, 1, the palsoozoic Tessclati, or Palmo-echinoidca, — with, 35 — 75 
radial zones of plates, mostly spheroidal in shape, with the oral and anal openings 
central, the former on the lower, the latter on the upper side, — and 2, the Eu-echi- 
noidca with only twenty radial zones of plates. This latter sub-order is divisible in 
the following manner : — 
EU-ECniNOIDEA. 
A. Endocyclica, round or angular, spheroidal ; mouth inferior, central, anus 
within the ajiical disc. 
{a) Apical disc, composed of ten plates surrounding the anal opening which 
is central ; jaws present. 
] . CwarWjE, amlnilacra mostly somewhat flexuous, narrow, with very small equal tubercles, 
or finely granulated; inter-amhulacra with large tubercles; peristome entire, circular or pentagonal. 
2. Diadem ATIDM, ambulacra slightly flexuous or straight, wide, provided with larger tubercles, 
as are also the inter-amhulacra; peristome with incisions, decagonal. 
3. EciliElDyE, amhulacral and inter-amhulaeral plates with numerous small tubercles, poriferous 
zones with more than two pores in a transverse series, often crowded ; peristome with incisions. 
{b) Apical disc composed of more than ten plates; anus central or slightly 
excentric. 
4. 5a LENIDJE, inter-amhulacra wide, with large tubercles. 
B. Exocyclica, test round or polygonal, spheroidal or mostly hemisphcroidal ; 
mouth inferior ; anus always separate from the apex. 
i). EciiiNOCOSlDJi ; poriferous zones comjdete, extending straight from the mouth and meeting 
above at the central apex in a point ; peristome notched ; jaws present. 
C 
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