CRETACEOUS ECHINODERMATA. 337 
Family Oputurips. 
Genus Opuiura, Linn., Ag. 
Ophiura serrata, Roemer. (Tab. XXIII. fig. 20.) 
This pretty species has been already described in the Geological Proceedings. 
It occurs in the white chalk, and was first observed in Germany. 
The specimen figured from the collection of the British Museum has a very 
perfect disc, showing the plates at the base of the arms ; found by Mr. Dixon in 
Kent and Sussex. 
SEA-URCHINS. 
The Echini or Sea-Urchins are radiated animals of a more or less globular, 
egg-shaped or disc-like form, whose bodies are completely encased with shelly 
plates, regularly articulated together, and so arranged as to leave conspicuous 
openings only for the mouth and vent, the former being invariably on the under 
side or base of the body, the latter changing its position in different genera. 
From the mouth to the vent, or from the mouth to the dorsal summit, or 
around the pores which open into the oviducts, and which are always on the 
dorsal surface, there are radiating avenues of pores called ambulacra, out of the 
perforations in which are protruded soft sucker-tipped tentacles or feet. The 
mouth is often armed with a complicated apparatus of teeth. On the surface of 
the plates covering the body are tubercles, bearing articulated to them spines of 
various sizes, sometimes very large and heavy. By the joint action of these 
spines and the suckers before mentioned, the Sea-Urchins can progress, in a 
creeping fashion, with considerable rapidity. Among these spines in many 
kinds are found curious, sensitive, minute pincer-like organs called pedicel- 
lari. The fossil Sea-Urchins are all constructed on the same plan with the 
living species, but there are several genera among them of which we have no 
living examples. 
In consequence of the untimely death of the lamented author of this work, 
the following catalogue of Echinide must be little more than a bare enumeration 
of the species which he collected in Sussex, or which were contributed for de- 
lineation by his friends on account of their beauty or novelty. Several of the 
