part 3] THEOUGH THE AINDES OF PERU AXD EOLTTTA. 267 



from the coal-mine of Pariatambo and incorrectly assigned by Gabb to the 

 Lias.^ According- to the description there given, the surface of the shell is 

 marked by numerous regular ribs, slightly flattened on their upper surface, 

 and having concave interspaces nearly as large as the ribs. This feature, 

 however, does not appear to be borne out by the figure, in which the type of 

 ornamentation seems to be very similar to that exhibited by the specimen 

 illustrated here (PL XVI, fig. 4). 



A further type from the same locality (Pariatambo), differing from that 

 just mentioned in the regular bifurcation of the ribs, is that described by 

 Prof. G. Steinmann as Schloenhaciiia acutocarinata (Shumard) Marcou, 

 variety ' multifida,' Steinmann thereby accepting Marcou's interpretation of 

 Shumard's type. A comparison of Steinmann's figure ^ with that of Shumard •* 

 shows that there is no justification for regarding the former as a variety of 

 the latter, since Shumard's type is distinctly costate at a diameter when 

 Schloenhachia miiltifida has closely-set flattened ribs. The Peruvian shell 

 may, therefore, be regarded as a distinct species. 



Steinmann also records other examples from Saco, between Oroya and 

 Pachachaca, near Yauh, the same locality as that from which my specimens 

 were obtained. If, then, I am justified in regarding these as topotypes, 

 Steinmann's figure calls for a certain amount of emendation. 



ScHLCENBACHiA MULTiFiDA Steinmann. (PI. XVI, figs. 2-5.) 



In the nature of the ribbing, degree of involution, and whorl-section, the 

 type agrees closely with my examples ; but it is clear that the specimen 

 illustrated is in the form of a cast, and the depth of the sulci is thereby 

 exaggerated."^ This is borne out by the fact that distinct ribbing is shown 

 on the inner whorls, whereas when the outer shell is preserved, it is, as can 

 be seen in my figure (PL XVI, fig. 2) on the left of the umbilicus, at this 

 diameter almost smooth or marked only by fine striae. 



Again, in the type-figure the ribs are indicated as widening gradually 

 towards the periphery, while in my examples the greatest breadth occurs on 

 the flank of the shell, where the forward curvature of the ribs commences, 

 whence they taper to their peripheral termination. This sickle-shaped 

 curvatvire, moreover, is but poorly shown in Steinmann's illustration. 



It was found that if a line be drawn joining the centre of a shell, of the 

 same diameter as the type, with the peripheral termination of any one rib, 

 this will cut as a rule five other ribs immediately anterior to it, as shown in 

 the subjoined diagram (text-fig. 4, A, p. 268) and also, though less clearly, 

 in PL XVI, fig. 3, at the point indicated by an arrow. [PL XVI, fig. 4 

 represents a portion of the lateral surface of the same specimen, and there- 

 fore does not show the peripheral curvature of the ribbing.] 



Further, in the forms collected by me the branching of the ribs takes place 

 nearer the umbilical margin than is shown in the type-figure. 



Despite these apparent differences I regard the specimens figured here 

 (PL XVI, figs. 2-5) as representative of Steinmann's species ^muUifida,' and 

 I am of the opinion that Ammonites carhonarixis of Gabb, if not identical, 

 is at least a closely-related form. 



One other feature remains to be noticed : namely, the peculiar character of 

 the siphuncle. This, in place of the usual circular section, has the outline 

 shown in the diagram : the shape being that of a laterally- compressed ellipse, 

 somewhat swollen at its inner end. This is not a chance feature due to 

 crushing, nor is it a mere infilling of the hollow keel above the true siphuncle, 

 but is a constant feature in all those specimens where the latter is visible 

 (see fig. 4, D, p. 268j. 



1 See Bibliography, § IV, No. 40, p. 269 & pi. xxxviii, figs. 2, 2 a, 2 h. 



- Neues Jahrb. vol. ii (1881) pi. vii, fig. 1. 



•' Op. supra cit. pi. iii, fig. 1. 



** A condition represented in my figs. 2 & 5. 



