part 3] THKOuciK the andes or peeu and bolitia. 271 



The morphogenetic stages of growth, may be summarized as follows : — 



Youth. — Smooth shell with falciform strias. 



Adolescence. — Shell with simple ribs extending continuously to a dorsal 



row of tubercles. 

 Maturity. — Shell with an inner row of tubercles developed by lateral 



thickening of the ribs (usually commencing at about the tenth). These 



are separated from the dorsal tubercles by a portion of the shell in 



which the ribbing is suppressed. 



In the peripheral views of adult forms given by both Gabb ^ and Lisson,^ 

 the ribs on the peripheral side of the dorsal row of tubercles are shown as 

 being prolonged towards the keel with a strong forward curvature. This 

 particular feature, however, is not exhibited by any of the specimens in my 

 collection, in all of which the ribs tend to die out on the dorsal surface. 



It was noticed in one example that at a comparatively late stage of growth 

 the ribs were continuous to the dorsal surface, where each expanded slightly 

 to form a single laterally-compressed tubercle, thus preserving the adolescent 

 character noted above. Occasionally, however, an individual rib may show 

 a slight elevation above its neighbours, though no lateral tubercles are 

 developed. In this respect the ornamentation is not unlike that of Amino- 

 nites delaruei D'Orbigny, to which, as suggested by Dr. Lisson, Scliloenhachia 

 ventanillensis probably is closely related. 



CENOMANIAN. 



Although there is but little variation in the lithological characters 

 of the limestones over the greater part of this district, it is probable 

 that more than one subdivision of the Cretaceous System is present, 

 for certain beds are met with at various localities which contain a 

 fauna, quite distinct from that recorded above, characterized by an 

 abundance of echinoderms and lamellibranchs, and by an almost 

 complete absence of ammonites. This fauna is regarded by 

 Dr. Lisson as being of Aptian age ; but, after comparison with a 

 closely-related assemblage of species described by Dr. Bose from 

 Mexico, and with many forms that appear to be their nearest 

 European equivalents, I am of the opinion that the horizon repre- 

 sented is above rather than below the Albian, and may with 

 more reason be assigned to the Cenomanian. At Kilometre 225, 

 between Oroya and Huari, on the Huancayo railway, these beds 

 were found overlying the typical ammonite-beds of the Albian. 



Such an occurrence cannot, however, be taken as definite strati- 

 graphical proof that they are of later date, for the district is one 

 that has been subjected to much disturbance, and inversion of the 

 strata would not be entirely unexpected. 



The chief forms that characterize these beds are comprised in 

 the following- list : — 



Enallastei' texanus (Roemer), 



Holectypiis planatus Roemer. 



Di2)lopodia hilli Clark. 



Diplopodia texana (Roemer). 



Ecliinohrissus subqtiadratus D'Or- 

 bigny. 



Pecten (NeitheaJ quadricostatiis 

 Sowerby. 



Pecten cf. chihuahuensis Bose. 



Liopistha (Psilomya) gigantea 



(Sowerby). 

 Plicatitla cf. inflata Sowerby. 

 Lima (Mantellum,) cf. m^exicana 



Bose. 

 Exogyra cf . conica Sowerby. 

 Modiola sp. 



Natica aff. collina Conrad. 

 Ttirritella granulata Sowerby, 



var. cenomanensis D'Orbigny. 



^ Op. cit. pi. xxxix, fig. 2 a. ^ Op. cit. pi. xvi, fig. 1 b. 



Q. J. a. S. No. 307. X 



