108 HORN EXPEDITION — PALAEONTOLOGY, 



Obs. — This fossil presents a remarkable analogy to Crassatella sulcata of the 

 Eocene of Paris and Barton. 



IsoaPCa opifoPmis, sp. nov. (Plate II., Figs. 16(j, 16(5.) 



Sp. char. — Cast quadrate, with a very large, strongly curved, sub-central 

 approximate beak \ the steep posterior slope is demarked by an elevated keel 

 extending from the umbo to the post-ventral margin ; the anterior side depressed 

 and projects forwards ; its margin is rounded. Hinge-line arched ; teeth transverse 

 and direct, about six on the anterior side, about twenty smaller ones on the 

 posterior side. The test must have been very thick, at least anteriorly, judging 

 from the large conical cast of the anterior adductor impression. Diameters : — ■ 

 Antero-posterior, 20 mm. ; umbo-ventral, 18 mm. ; sectional, 18 mm. 



Loc. — In quartzite, between Petermann Creek and Tempe Downs. 



The only important variation in shape exhibited by this species is in the width 

 of the posterior area ; when at its shortest the umbonal keel hides it when viewed 

 from above. An extreme form has that area wider, therefore not so steep and 

 so concealed from view by the umbonal carination. The species name is in allusion 

 to the external resemblance this fossil has to some species of Opis, e.g., lunulatus. 



IsoaPCa wattii, sp. nov. (Plate II., Figs. 17«, 17*^.) 



Sp. char. — Cast oval-oblong to cuneiform, with very large, strongly curved, 

 sub-marginal, approximate umbones, rounded in front and attenuated behind ; 

 ventral margin straight, post-dorsal margin arched. The post-dorsal area is 

 declinous, arising from the umbonal ventricosity extending to the post-ventral 

 angle, so much so that the hinge-line is posteriorly hidden when the cast is 

 viewed from above. Teeth as in /. opijorniis., few and large anteriorly, numerous 

 and smaller posteriorly. Diameters : — Antero-posterior, 25 mm. ; umbo-ventral, 

 16 nnu. ; a large example, 32 mm. by 20 mm. 



Loc. — In quartzite between Petermann Creek and Tempe Downs, associated 

 with /. opiforinis, from which it differs by its cuneiform shape. 



Palaeai^ca wattii, sp. nov. (Plate IT., Fig. 14.) 



Sp. char. — Shell transversely oblong to obliquely rhomboid, moderately ventri- 

 cose; umbones almost marginal, incurved and approximate; anterior much narrowed; 

 posterior side much expanded, obliquely truncated. Surface of cast marked by 



