FOSSILS OF THE CLINTON GROUP. 583 



The muscular impressions of the brachial valve are always separated 

 by a median ridge, which may be low in young specimens but which is 

 strong, though rounded, in older individuals. This ridge extends back- 

 wards, with about equal width, to its junction with the cardinal process, 

 there being often no well defined line of demarcation between it and the 

 latter. This, in young specimens, is about the only marking to be seen 

 over the muscular area ; but in older specimens, owing to the general thick- 

 ening of the shell, especially posteriorly and towards the beak, except 

 over the muscular impressions, this muscular area receives a distinct 

 boundary postero-laterally, which becomes prominent antero-laterally, 

 and which can be quite distinct or almost absent anteriorly. When best 

 developed it has a rather round outline, with a slightly heart-shaped 

 anterior margin at the termination of the dividing median ridge. It is 

 further divided into anterior and posterior impressions by narrow ridges 

 connecting the median ridge with the lateral margin of the area. These 

 lateral ridges are rarely distinct, so that the anterior and posterior adduc- 

 tor impressions are rarely clearly defined from each other. The anterior 

 adductor scars are a little larger and wider. Vascular markings were not de- 

 tected unless faint furrows crossing the anterior border of the muscular area 

 and a few indefinite markings anterior to the median part of the muscular 

 area be so interpreted. 



The muscular area of the pedicle valve is distinctly bounded 

 postero-lateraily by the general thickening of the shell posteriorly 

 around the muscular area. While this thickening, where it adjoins 

 the muscular area and bears the vertical plates that serve to support 

 the teeth, is well marked posteriorly, it disappears so rapidly anteriorly 

 in some shells, that even the lateral boundaries cannot be said to 

 be well defined, whereas the antero-lateral boundaries are commonly 

 indistinct. This being the case, it may be imagined how rarely the ante- 

 rior margin of the muscular area, which depends for its definition upon 

 a similar thickening of the shell anteriorly, is well defined. When best 

 defined the muscular area of the pedicle valve is seen to resemble in out- 

 line that of Orthis testudinaria, It has namely, laterally, a diverging 

 outline for a short distance anterior to the teeth, after that a converging 

 outline, after which the outlines on either side of the muscular area 

 become parallel, as far as the antero-lateral angles of the area. Ante- 

 riorly the area has a more or less obversely V-shaped outline (a), the 

 antero-lateral angles being more or less rounded or acute. At the point 

 where the converging lateral outlines of the area assume a parallel direc- 

 tion, there is found on either side the termination of a sort of low 

 ridge which serves to divide the muscular area into anterior and posterior 

 diductor muscular impressions. These divergent ridges are however 

 rarely developed. Along the median line of the muscular area, between 

 the impressions on either side, there is anteriorly at times as light, or more 



