Fauna of the Morrow Group 
153 
above the ears but anteriorly the venter^ is fairly sharply de- 
fined from the main flanks and the outline of the valve towards 
its margin is distinctly subquadrate as a result of the flattening 
of the venter and main flanks. A shallow, rounded, undefined, 
mesial sinus originates on the umbo a short distance from the 
beak and extends to the anterior margin ; on the venter it neither 
expands nor deepens notably and it is in most cases a rather 
inconspicuous feature of the shell, in some individuals the sinus 
is practically absent while in others it is moderately deep, con- 
siderable variation in its width also occurring in different shells. 
Surface of entire valve with the exception of the cardinal ex- 
tremities marked by fairly strong, rounded costae^® crossed by 
concentric ribs in the visceral portion of the valve and not 
rarely modified by the bases of erect tubular spines scattered 
irregularly over the surface. The costae increase in number 
in the visceral region by bifurcation or implantation and on 
the venter they may in some instances coalesce, with a corre- 
sponding decrease in number and increase in size; the coales- 
cence normally takes place at the base of a spine where two 
or three costae unite to form one stronger costa which con- 
tinues from the spine toward the anterior margin. In the an- 
terior part of the visceral portion of the valve the number of 
costae in a 10 mm. space is typically 17 or 18, but may be as 
few as 14 or as many as 21, The concentric ribs of the visceral 
region are bent inward and compressed toward the cardinal 
margin but may again be deflected outward in crossing the ears 
where they are less strong and more closely crowded than on 
the umbonal slopes; there are generally 10 or 12 ribs in the 
space of 10 mm. in the umbonal region. Spines may occur 
anywhere on the shell surface but there is always a double row 
of spine bases which originates in the depression between the 
cardinal slopes and the ears and curves forward in a line 
roughly parallel to the surface of the venter and occupies the 
median portion of the main flanks. In this double row the large 
hollow spines ordinarily alternate in position and are normally 
*“The pedicle valves of the Producti are often very convex or globose. 
In such cases the lateral sloping surfaces anterior to the cardinal and um- 
bonal slopes may be termed the main slopes or main flanks, while the por- 
tion lying between these main flanks and extending from the umbo to the 
anterior extremity may be called the venter.”'^'^ 
