SOME NEW DEVONIC FOSSILS 239 



point. In form and nearly every detail of outline, surface and 

 contour, in musculature, cardinal arrangement, brachial structure so 

 far as known and in intimate shell structure they are homogenic. 

 In Amphigenia however the converging dental plates do not reach 

 the bottom of the valve but first unite and the resultant spondy- 

 lium is supported on a short median vertical septum. In Rensse- 

 laeria the plates converging, fail to unite but meet the inner wall 

 of the shell leaving between them a narrow surface, which is in 

 effect the base of the spondylium. In this special feature which 

 can hardly be accredited with high value as an anatomical differ- 

 ential, there is a definite indication of progress. The Gaspe shells 

 show how frail is this conventional distinction. The convergence 

 of the dental plates leaves only a very narrow space between and 

 quite frequently they come together at the very surface of the shell 

 wall. Even a single vertical septum may develop in the later forms 

 of the Gaspe sandstone. It is natural to compare the small elon- 

 gate shells from Gaspe with Hall's R. marylandica from the 

 Cumberland Oriskany. They are shells of the same proportions 

 but in respect to development of the dental lamellae the latter is 

 rather less progressed than R. ovoides. 



In view of the evidence presented by these Gaspe shells it seems 

 to us very desirable to regard Amphigenia essentially synony- 

 mous with Rensselaeria and indicating as we have said a progressed 

 condition of one feature only. Rensselaeria has many specific 

 expressions and among the forms now referred to it are several 

 more significant departures than that presented by A. e 1 o n g a t a . 



Lower Devonic. Grande Greve and Perce. 



Middle Devonic. Gaspe Basin, P. Q. 



Rensselaeria stewarti nov. 



Shell naviculate, the unequal convexity of the valves being very 

 marked. The ventral valve is highly convex and arched, the line 

 of greatest curvature being median from which the slope is some- 

 what abrupt to the sides giving the valve a subcarinate exterior. 

 The umbo of this valve is high and overarched, projecting far 

 beyond the hinge line, the apex being incurved and truncate. The 

 cardinal area is represented by a flattened triangular area free of 

 striae and rather definitely delimited. The dorsal valve is gently 

 and evenly convex with low and inconspicuous umbo and beak. 

 The surface of both valves is covered by abundant subequal radial 

 riblets all of which are simple and continuous from beak to margin 



