part 3] GEOLOGICAL SECTIOJs^S THROUGH THE AXDES. 269 



In 1912 a revision of Steinmann's Peruvian material was made by O. Schlag- 

 intweit^ which resulted in the groviping together of a number of widely -differing 

 forms under the title of A. d'Orbigny's species ' Schloenhachia royssiana,' and 

 again in 1913 they were once more united by Dr. C. I. Lisson as Schloenhachia 

 peruana Von Buch. That such drastic treatment was quite unwarranted is 

 apparent from the number of distinct forms represented in my o"\vn collection. 

 Of these several may be tentatively referred to types figured by Dr. E. Bose 

 from Mexico.- Here are included species with widely-spaced costae, the 

 outer shell of which, unlike that of ' multifida,' conforms closely with the 

 internal corrugations shown by the cast (compare the cross- sections of the 

 shells illustrated in fig. 4, B & C, p. 268). 



In most of the forms possessing broad sulci common to both shell and 

 cast, the posterior faces of the ribs have a gentle slope with a convex ciu'va- 

 ture ; while the anterior faces are, as a rule, steep, and either plane or slightly 

 concave (see PL XVII, fig. 1). In ' multifida,' where the ribs are flattened 

 and close-set, the underlying ribs of the cast have an almost rectangular 

 cross-section. 



ScHLCENBACHiA sp. (cf. ACUTOCAKiNATA Bosc uon Shumard). 



The example illustrated in PL XVI, fig. 1, although closely related to 

 ' multifida,' differs from it in the possession of more widely spaced ribs 

 with less regular dichotomy. At the margin of the almost vertical 

 umbilical wall, the ribs, moreover, have a pronounced backward sweep ; 

 whereas in ' multifida ' they are not only almost radial in direction at 

 their origin, but about twice as numerous for the same length of margin. It 

 appears to have a certain resemblance to the form figured by Dr. E. Bose as 

 ' aff. acutocarinata ' Shumard ^ ; but this again appears to differ very widely 

 from Shumard' s original type, and whether Bose is correct in his interpre- 

 tation of the latter is extremely doubtful. His figured example is, like mine, 

 in the form of a cast, but traces of adhering shell suggest that the true 

 ornament is of the open-ribbed type, intermediate between ' m,ultifida ' and 

 ' helTcnapi ' (see below). A comparison of the suture-lines shows that the 

 lateral lobes are somewhat deeper and narrower in the Peruvian form, and in 

 this respect it more closely resembles Schloenhachia cf. helkna'pi Bose {non 

 Marcou).* 



Other forms with still more widely spaced ribs are also represented in my 

 collection. These are more evolute in character, and possess simple ribs 

 with (in general) a less-pronounced forward curvature. One example 

 (PL XVII, fig. 3) characterized by thin, sharp ribs, at first almost straight 

 and then expanding rapidly where they bend forward near the periphery of 

 the shell, appears to be analogous to the Mexican species, Schl. chihuahuensis." 

 In Bose's description of the type the ribs are stated to curve strongly forward 

 in the upper quarter, diminishing in height to disappear near the keel ; but, 

 except in the peripheral view (pi. viii, fig. 1) which agrees closely with 

 mine, this curvature is not clearly shown in the illustration (pi. vii, fig. 3), 

 where the ribs appear to be almost straight throughout their length.^ 



ScHLCENBACHiA cf, BELKNAPi Bosc (non Marcou). 



The most pronounced development of ribbing is that shown by the specimen 

 figured in PL XVII, fig. 1, which represents a fragment of the cast of a large 



1 See Bibliography, § IV, No. 50. 



^ ' Monografia Geologica & Paleontologica del Cerro de Muleros ' BoL Inst. 

 GeoL Mex. 25 (1910). 



3 I hid. p. 65, pL i, fig. 3 & pi. ii, figs. 1-3. 



■* See E. Bose, op. cit. pi. vii, fig. 5. 



" Id. ihid. p. 73 & pi. vii, fig. 3, pi. viii, fig. 1. 



^ Compare J. Marcou's figure of Arnmonites gibhonianus Lea, ' Geology of 

 North America ' 1858, pi. ii, fig. 2 a. 



