EARLY DEVONIC HISTORY OF NEW YORK AND EASTERN NORTH AMERICA 1 65 



Length of ri full grown shell usually about two and a half inches; width, twelve to 

 fifteen lines. Sometimes the depth of the two valves together is greater than the width. 

 The shell is often more or less compressed laterally, and exhibits distortions, -which are 

 natural, or not the result of pressure. 



Analyzed as to exterior characters this shell is a miniature of the great 

 Renss, ovoides of the New York Oriskany, varying in proportion, 

 dimensions, outline and convexity, much as that shell does, that is, frequently 

 high shouldered and broad across the umbones, rarely broadest in the pal- 

 lial region, often with lateral margins vertical or slightly introverted specially 

 about the umbones, but as often without this character, usually with the 

 ventral valve medially elevated, and finally with a diversity in the character 

 of surface striation which is due to the fact that the fine striae of early age 

 maintain their simplicity but increase in width without additions, to that in 

 old shells or progressed stages of relatively young shells where the surface 

 may seem to be coarsely marked. On the other hand the shells are charac- 

 terized by a prevailing narrow elongate-linguate outline with parallel lateral 

 margins for a great part of their length. On the interior there are a few 

 notable and perhaps no constant differences, whether in respect to struc- 

 ture or musculature or cardinal plate. It is however here important to 

 bring forward the fact which the writer has already expressed with some 

 emphasis, that as between the genera Rensselaeria (as based on the type 

 species R. ovoides) and its chronologic successor Amphigenia, there is 

 a distinction solely in one structural 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 spondylium 

 is supported on a short median vertical septum. In Rensselaeria 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 differential, there is a definite indication of progress. The 

 Gaspe shells show how frail is this conventional distinction. The converg- 

 ence 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 sand- 

 stone as shown in our figures. It is natural to compare the small elongate 

 shells from Gaspe with Hall's Rensselaeria 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 e n s s, o v o i d e s. 



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

 necessary to regard Amphigenia essentially synonymous with Rensselaeria 



