S86 
PALAEONTOLOGY OF NEW-YORK. 
abruptly to the sides : the usually undefined mesial fold becomes 
visible below the middle of the valve, and usually Very conspicuous 
towards the front, which is abruptly elevated. 
Surface marked by concentric lines of growth, and the lamellae often 
extending and closely imbricated; fine interrupted and scarcely dis¬ 
tinct radiating striae, which appear like ducts within the substance of 
the shell, mark the surface in many specimens. The upper part of the 
shell is sometimes nearly free from imbricating lamellae, but they 
become crowded towards the front. 
The interior of the ventral valve shows two strong teeth at the base 
of the foramen, extending in strong plates to the base of the rostral 
cavity, which is distinctly hollowed out for the reception of the pedicle. 
The muscular impressions begin jult at the base of the rostral cavity, 
and continue to a little below the middle of the length of the valve, as 
shown in fig. 27. These features are impressed upon the cast, as shown in 
figures 29 and 30 ; and the surfaces outside the muscular impression are 
often beautifully and variously marked by vascular impressions which 
are numerously ramified towards the margin, as shown in figure 30. 
This character is often more extreme, and the ramifications are at least 
tAvice as numerous as in this figure; a feature more faintly shown in the 
lower part of figure 29. 
The interior of- the dorsal valve is furnished with a strong cardinal 
process, the centre of which is often depressed and somewhat spoon¬ 
shaped, but frequently thickened : it is bounded posteriorly by elevated 
socket-margins, and the crural bases diverge from either side of the 
anterior portion. The muscular impression is narrow elliptical, and 
divided by a low median crest. These features are shown in figure 26; 
and the reverse or impression is well preserved in figure 28, which is a 
cast of the interior of another dorsal valve. 
The internal spires are often well preserved, and show about fif¬ 
teen turns : the two bands are connected at a point about half the length 
of the shell from their commencement, by a sharply arched loop, in the 
manner shown in the illustration given, and which is indicated in fig. 31. 
