PALEONTOLOGIC CONTRIBUTIONS IOI 



carbonaceous and was doubtless horny in the living animal, while 

 the aptychus consists of two thick calcareous plates. The anap- 

 tychus is, however, now considered as having been a conchiolinous 

 plate that always was attached to the interior side of the aptychus 

 and which by its elasticity may have fulfilled the function of the 

 ligament of the shell of the lamellibranchs (see Steinmann- 

 Doderlein, Elemente der Palaontologie, p. 387. 1890). Since in 

 the Paleozoic rocks no aptychi have been found and the formation 

 of shells generally begins with the conchiolinous deposits which 

 later become more or less calcified, it is quite proper to assume that 

 the Paleozoic goniatites may not yet have had advanced to the" 

 calcareous portion of the aptychus structure. It is in this connection 

 worth while to consider the different views on the formation of 

 the aptychus. They have been considered as opercula and as covers 

 of the nidamental gland. The position of the aptychus in the conch, 

 its form and convexity however, as well as the fact that it sometimes 

 has a shrunken appearance, indicate that it is a calcified pair of 

 cartilages, to the inside of which muscles were attached. Its normal 

 position corresponds to that of the base of the funnel of the animal, 

 whose muscles are also in Nautilus attached to a noncalcified pair of 

 cartilaginous plates (Steinmann-Doderlein, op. cit. p. 388). 



If the development of the aptychus started from a cartilaginous 

 body whose interior surface became first conchiolinous, the extreme 

 thinness of the valves of Spathiocaris and the other Discinocarina 

 is thereby explained, as well as the remarkable flexibility which 

 shows itself in the bending and frequent doubling of the shells upon 

 themselves. Furthermore, the extreme variation in the shape and 

 depth of the anterior notch which varies from a shallow emargi- 

 nation (see Pal. N. Y., v. 7, pi. 35, fig. 13) to a deep cleft {op. cit. 

 pl- 35> fig- T 5) even in the same species, and which is difficult to 

 explain in a crustacean, and especially the narrow cleft in the pos- 

 terior portion of Dipterocaris which looks strange for a carapace of 

 a crustacean (see op. cit. pi. 35, figs. 20—26) are readily accounted 

 for by reference to a partially conchiolinous aptychus. The last- 

 mentioned notch or cleft would indicate the gradual coalescence of 

 the two halves of the funnel, supposed to have been a process of 

 some phylogenetic importance in the cephalopods. Also the pecu- 

 liar median longitudinal lines upon the valves of Spathiocaris and 

 the diverging median lines upon the valves of Pholadocaris find 

 their counterpart in similar lines along the median hinge-line of 

 the aptychus halves. 



