494 TRANSACTIONS OF THE WAGNER FREE 



lated to the roughened area in Venus niercenaria than to any other part of a 

 Teleodont hinge, and even if dynamically equivalent, morphologically are not 

 homologous with Teleodont cardinals. 



Another absolutely distinct type of hinge is that which Fischer has named 

 Isodont. This is exemplified in the genus Spondylus and a few related forms. 

 Here we have an internal resilium with (in the left valve) first a socket and 

 outside of that a strong tooth, on each side of the pit ; and, in the opposite 

 valve, corresponding teeth and sockets. No other group has the teeth and 

 sockets concentrically arranged and if this distinction from their obviously 

 near relatives, the Pectens, were morphologically true, we should find 

 it difficult to maintain the value of dental types for subdivisions higher than 

 sub-families. It is through a study of the Pectens and very young Spondyh 

 that I have been able to satisfy myself that the so-called teeth of Spondylus 

 (and of some Pectiniform relatives) are not homologous with either Prionodont 

 or Teleodont teeth, but are a modification of certain ridges which reinforce 

 the auricles in many Pectens and which I named, in 1886, the auricular crura. 

 I have been able to trace this modification of the crura to a point where it 

 absolutely harmonizes with the incipient teeth of Spondylus 3.t an age when the 

 original area-like teeth of Pecten and Spondylus still occupy the cardinal mar- 

 gin. Even in adult Pectens the traces of the true teeth sometimes remain and 

 I have found them quite emphatic even on some well-grown specimens of 

 Spondylus. Consequently we must regard the hinge of Spondylus as a remark- 

 able specialization of certain features of the shell, themselves the result of 

 modifications of the valves initiated through the influence of external sculp- 

 turing, but morphologically distinct from the structures which in other bi- 

 valves we call teeth.* 



Among the Prionodesmacea, after the Taxodonts and Schizodonts are dis- 

 posed of, dismissing to the former those Paleoconchs which, like PrcBcardium 

 and Pleurodonta, seem to illustrate the evolution of the Taxodonta, there still 

 remains a group that illustrates the development of the Heteromyaria repre- 

 sented in the recent fauna by Crenella. These are Prionodont forms in which 

 the valves became markedly inequilateral, so that the anterior cardinal margin 

 was much shortened, and also, by simultaneous umbonal torsion, the rib end- 

 ings and their dental processes became curved. A conspicuous modern exam- 

 ple of this type is the little Crenella to which Orbigny gave the name Njiado- 

 cardia. All the Mytilidce have hinges of this type, the AvicididcB being Schizo- 



*Since arriving at this conclusion I recall that Jackson was led to the same opinion from his studies of 

 young Priouodonts ; Jlem. Bost. Soc. Nat Hist. vol. iv, p. 3'JO, ISOO. It should be uoted that the species are not 

 alike in retaining the archaic denticulations either in Pecten or Spojidyltis. I found them in a specimen of a 

 red Spondylus, 4 mm. long, from the West Indies, and S. Gussoni sometimes shows them ver.v clearly. Among 

 the Peciinidx P. {Pseudamusium) fhalassinus Dall, not only shows these teeth plainly in the adult, but up to the 

 time the young are half grown the teeth are functional, as much so as in Nuciiia, and even when the soft parts 

 and resilium are absent the thin glassy valves when opened will sometimes break before the teeth release 

 them, so efTectlvely do they interlock. Cf. Blake Pelecypoda, Bull. Mus. Comp. Zool. xii, p. 221, 1S86. In 

 quite a number of species no teeth are found. 



