1894. 



PROCEEDINGS OF THE NATIONAL MUSEUM. 95 



My niucli regretted frieud, the late Dr. Paul Fisclier, in his Manual 

 has compared Rangia to a Cyrena with an internal cartilage, and has 

 regarded the cardinal teeth of the former as alternating, or Heterodont, 

 and those of Mactra as of the tyi)e Avhich has been called Desmodont 

 by Neumayr. For these reasons he plac<Kl his family Rdugiida' immedi- 

 ately after Cyrenidw in the Manual. If he had been able to study the 

 series which has l)een available for me I can not doubt he would have 

 changed this opinion. A study of the young shows that the hinge of 

 Gnathodon in its early stages is as typically Desmodont as that of 

 Mactra and that the truncation of the ^-shaped teeth is a dynamic 

 feature due to the exigencies of growth, which may be observed in 

 Mulinia as well as Gnathodon. As a matter of fact neither jlfactra nor 

 Gnathodon has genuine Desmodont dentition. The hinges of both are 

 really Heterodont. 



In the young Gnathodon cnneatns 10 mm. long, the hinge possesses 

 the following armature: 



Left valve : Anterior lateral tooth slender, slightly arched, crenulate 

 above, behind without the characteristic hook from which Gray derived 

 his name for the genus; cardinal tooth thick, y\-shaped, with a pro- 

 nounced depression on each side of it; anterior border of the cartilage- 

 pit with a small accessory lamella; the upper part of the anterior 

 border showing a small blunt projection corresponding to the hiatus 

 between the cartilage and the ligament above; this is jirobably a relic 

 of the shelly bridge which roofed the pit before the ligament descended 

 into it; pit deep, its ventral border projecting as in .1/«c#ra; the inser- 

 tion scars of ligament above and cartilage below entirely separate, with 

 a small shelly ridge rising between them; posterior lateral long, thin, 

 slender, arched, crenulate above. 



Eight valve: Furrow for the anterior lateral tooth narrow, crenulate 

 on both sides, the lamina below it not much thickened; cardinal teeth 

 two simple lamellai closely approximated (but not joined) at their upper 

 ends, with a ^-shaped pit below them, into which is received the car- 

 dinal tooth of the opposite valve; (this arrangement is exactly paral- 

 leled in Midmia lateralis); posterior groove for the lateral of the left 

 valve narrow, crenulate on both sides; the lower lamina slightly more 

 prominent than the upper one; other features as in the left valve. 



At this stage the pallial sinus is proportionally larger and rounded 

 anteriorly as in Maetra, in short all the distinctive characters of the 

 young shell, in which it differs from the adult, are Mactroid. 



Looked at from the standpoint of dynamic evolution, the hinge of 

 this group and the other Mactridw in its develoiiment offers much that 

 is of interest. The various stages of immersion of the ligament in the 

 different genera and subgenera illustrate well the manner in which 

 it has been ingulfed. So too the changes between the juvenile hinge 

 and that of the full-grown adult when regarded from a dynamic stand- 

 point are more easy of explanation than from any other ])oint of view. 



