94 Dr. T. Wright on the Cassidulidse of the Oolites. 



Locality and stratigraphical range. — This rare species was 

 collected from the Corubrash at Stanton, Wilts, and is the 

 only specimen we know from that stratum. We have seen a 

 Pygaster from the Great Oolite near Cirencester, which appa- 

 rently belongs to our species ; it was a crushed specimen, but 

 the number of the tubercles on the area? was the same as in our 

 species. 



Genus Holectypus, Desor. 



Test circular, more or less hemispherical, conical or subco- 

 rneal, always tumid ; mouth central and decagonal ; anal opening 

 large, inferior, infra-marginal, rarely marginal, sometimes occu- 

 pying the entire space between the mouth and the border ; am- 

 bulacra simple, continuous and radiant ; avenues with a single 

 pair of pores throughout ; tubercles small, perforated and cre- 

 nulated, disposed in a regular series ; apical disc central and 

 vertical, composed of four perforated and a single imperforated 

 ovarial plate, a central madreporiform body, and five ocular 

 plates ; the internal walls of the test without projecting ribs. 



The genus Holectypus was formed by M. Desor for those 

 Discoidece which have no projecting processes or ribs on the inner 

 wall of their test. The species of which the group is composed 

 are found in the oolitic and lower cretaceous rocks. They con- 

 stitute a transition from the Discoidece to the typical Galerites, 

 and according to the views of Professor Forbes " form a section 

 or subgenus of the genus Galerites, more valuable on account of 

 their palaeontological merits and limited distribution in time 

 (being the main characteristic of the Oolitic period) than for the 

 zoological importance of the characters of their organization, 

 which are rather transitional than distinctive." 



Holectypus depressus, Desor. 



Syn. Galerites depressus, Lamk. Animaux sans Vert, tome iii. p. 21. 



no. 7 ; Desmoulins, Table Synopt. p. 254 ; Goldfuss, Petrefact. 



Ger. tab. 41. p. 129 ; Koch and Dunker, N. D. Oolit. tab. 4. 



fig. 2. 

 Discoidea depressa, Agassiz, Cat. Syst. p. 7 ; Echin. Foss. de la 



Suisse, tab. 13 bis, fig. 7-13. 

 Holectypus depressus, Desor, Catal. raisonnedes Echinides, A. S. N. 



torn. vii. 3rd Series. 



Test hemispherical, more or less depressed, sometimes conical ; 

 circumference circular or subpentagonal ; base concave ; tuber- 

 cles small on the dorsal surface, larger on the base; anal 

 opening pyriform, infra-marginal ; apex directed towards the 

 mouth. 



Height r 7 n ths of an inch, antero-posterior diameter 1 inch and 



