270 ME. J. A. DOUG-LAS OX CxEOLOGICAL SECTIONS ^Yol. Ixxvii, 



iiKiiyiduaL The deep sulci are here considerably broader than the ribs, 

 and the latter are characterized bv steep anterior faces which may be likened 

 to a series of escarpments. Fnrther. although the growth-lines of the shell, 

 when this is preserved, show a strong forward curvature near the periphery, 

 the ribs themselves, especially in the cast, run almost straight towards their 

 termination. 



This form appears to bear a close resemblance to Schlcenhachia belJcnapi 

 (ilarcou) as figured by Bose.^ whose interpretation of the type, however, is 

 somewhat different from that of its original author. 



Since Bose's illustration of the suture-line of a iTexican example is almost 

 identical with that of one of the closer-ribbed specimens figured here 

 (PL XTT. fig. 1). it is probable that these coarsely -ribbed shells represent 

 phylogenetic old-age forms of a close-ribbed stock. 



The small individual figured here (PL XVII, fig. 2) is possibly a youthful 

 form of SchJ. beJlinapi. The ribs are here seen to be relatively widely spaced, 

 even at a very early stage of growth. 



DiPLOCEEAS coEXVTirx (Pictet). 



The occurrence of this ammonite is of considerable interest, as pointed out 

 to me by Dr. L. P. Spath, in that it affords evidence for assuming the horizon 

 of the beds to be just below the level of the ' cristatum ' zone. 



The species is represented in my collection by a well-preserved almost com- 

 plete LaK of a shell having about the same diameter as the type. P. J. Pictet's 

 original figure - shows the latter as possessing smooth inner whorls ; whereas 

 in my specimen these are distinctly costate, even at a very early stage of 

 growth. In his peripheral view, moreover, the costae run at right angles to 

 the keel, while in the Peruvian shell these have a pronounced forward curva- 

 ture. In other respects, it agrees closely with the type. 



ScHLCEXBACHXA- TEXTAXiLLEXsis (Gabb). (PL XYII. fig. 4 & PL XT. 

 figs. 8 a-8 &.) 



The original type of this species was described by Gabb in 1877, and it was 

 then correctly stated that, while in the young form the ornamentation con- 

 sisted solely of fijie striae, these were replaced in the adult shell by strong 

 simple costae, each giving rise to a pair of pronounced tubercles, the change 

 being comparatively sudden, and taking place at about a diameter of 1 inch. 

 The species was, at that time, incorrectly assigned to the Lias. In 1881, 

 Prof. Steinmann redescribed a young smooth- shelled example from the same 

 localirv- as a new genus, to wliich he gave the name ^lojsisovicsia diiijeldi. 

 Later, in rectifying the error, he sought to prove the identity of the Peruvian 

 species with Araraonites inflatus Sowerby. A comparison with Sowerby's 

 original type-specimen, however, shows that the two are quite dissimilar. 



Reference must also be made to a number of examples figured by Dr. C. I. 

 Lisson.""* One specimen shows the transition from the smooth to the costate 

 stage taking place at a very early period of growth (j)l. xvi, fig. 4), the 

 adult type of ornamentation being fully de^'eloped at a diameter when the 

 numerous examples in my collection are still in the smooth stage (compare 

 PL XT, fig. 8, which may be taken as an average specimen). 



In the young forms figured by Dr. Lisson (pi. xv) the back is described 

 as being • rounded, with insignificant keel ' ; but, as is clearly shown by the 

 specimen illustrated here (PL XT, fig. 8 b), the latter often becomes pro- 

 nounced at a comparatively early stage, and its appearance is usually 

 accompanied by a slight flattening of the dorsal surface on each side. 



1 Bol. Inst. Geol. 3Iex. 25 (1910) pis. vi & vii. ;3^£ v- 



- • 3Iollusques Possiles .... dans les Gres Terts des Environs de Genere 

 pt. i, Cephalopodes (1847) p. 93 & pi. viii, figs. 6a-6c. 

 3 ' Ammonites del Peru ' Lima. 190S, pis. xv & xvi. 



