19G Dr. T. Wright on the CassiclulicUe of the Oolite. 



Affinities and dijferences. — In its general outline if. gibherulus 

 resembles the young of H. agaricifonnis, but its anterior gibbous 

 crest distinguishes it at a glance from that species, and I know 

 of no other form for which it could be mistaken. 



Locality and stratigraphical range. — This is a rare Urchin : I 

 know only four British specimens, the best of which, from the 

 cabinet of W. Walton, Esq., of Bath, has served for the foregoing 

 description, and to whom I beg to record my thanks for his 

 courtesy in forwarding it. This Urchin was collected by that 

 gentleman from the Inferior Oolite in the parish of Charlcomb 

 near Bath, from whence three of the four specimens were ob- 

 tained ; the fourth was found in the Inferior Oolite of Dorset- 

 shire, and is in the Museum of Practical Geology. 



History. — First figured and described by M. Agassiz, who 

 only knew of two specimens from the Inferior Oolite of Switzer- 

 land ; afterwards by M. Desor in his valuable monograph on the 

 Galerites, and now described as a British fossil for the first time*. 



Genus Dysaster, Agassiz. 



Test ovate or subdiscoidal ; ambulacra simple, continuous and 

 radiant ; the posterior pair separated from the others, and con- 

 verging to form a summit at some distance behind that formed 

 at the apical disc by the antero-lateral pair and anterior single 

 ambulacrum which gives value to the name Dysaster. Upper 

 surface of the test smooth and convex, under surface much undu- 

 lated from the convexity of the interambulacra and the straight- 

 ness of the ambulacra. Tubercles small, mammillated and per- 

 forate, and surrounded by a circular depression ; apical disc situ- 

 ated at the junction of the three anterior ambulacra, and formed 

 of four perforated ovarial plates which are intimately soldered 



* After the first part of this paper was printed, I met with a specimen of 

 Hyboclypus caudatus with the apical disc preserved in situ, which I regret 

 was not found before the plate containing the details of the anatomy of 

 that species was completed. The disc is formed of two small anterior ovarial 

 plates, the right plate supporting the madreporiform body, and two larger 

 posterior ovarial plates ; between them and occupying the centre of the 

 disc are four small rhomboidal plates, which probably represent the single 

 ovarial plate composed of two valves, and the two posterior ocular plates 

 displaced from their normal position in consequence of the posterior pair 

 of ambulacra terminating in this genus at a short distance behind the apical 

 disc ; at the summits of the single and anterior pair of ambulacra are three 

 small ocular plates with distinct eye-holes ; the posterior ambulacra have no 

 ocular plates at their summits ; these elements according to my view are 

 transposed to the centre of the disc to give greater expansion to the struc- 

 ture in this region. 



Pygaster Morrisii I have dedicated to my friend John Morris, Esq., 

 one of the learned authors of a Monograph of the Mollusca of the Great 

 Oolite. 



