RETICULARIA 433 



or in the umbonal region, ill-defined laterally, rounded on top, low or of 

 moderate height ; beak short and incurved, the umbonal region extending 

 a little beyond the hinge-line; cardinal area small, lying in nearly the 

 plane of the valve, vertically marked by striae similar to those of the 

 opposite valve. Internally the anterior face of the cardinal process is 

 flattened or concave, and is marked by fine vertical grooves, being 

 scarcely differentiated, except by these markings, from the surface of the 

 strong crural plates which flank it on either side, anteriorly from the 

 cardinal process a well-defined median septum extends for one-third, or 

 nearly one-third, the length of the valve; the muscular scars not well 

 defined; the deeper portion of the valve marked by radiating costae 

 similar to those in the opposite valve. 



The surface markings of both valves consist of regular, concentric, 

 imbricating rows of fine, double-tubed spines, the bases of these spines 

 are continued posteriorly upon the surface of the shell as slight ridges 

 which reach to the next adjacent concentric band. When partially 

 worn, the markings frequently appear to consist of rather regular, con- 

 centric bands, crossed by much finer radiating costae. 



Remarks. This species is a close ally of R. pseudclineata, but may be 

 distinguished from it by its proportionally narrower form, its more 

 strongly developed mesial fold and sinus, the less strongly developed 

 internal radiating costae, and the somewhat more remote concentric rows 

 of rather coarser spines upon the external surface. In the typical ex- 

 pression of each species they are not easily confused, but an occasional 

 example of R. pseudolineata with more strongly marked fold and sinus 

 resembles this species, but such specimens are commonly proportionally 

 wider. 



The species is typically developed in the Chester faunas, but it occurs 

 not uncommonly in beds as low as the Salem limestone, but these earlier 

 representatives are often of larger size. The dimensions which have been 

 given are those of a Chester specimen of about average size, but in the 

 Salem limestone the species sometimes attains a length of 30 or 40 mm., 

 being as large as R. pseudolineata. 



Horizon. Salem limestone, Chester group. 



RETICULARIA SALEMENSIS n. sp. 

 Plate LXXV, Figs. 15-19 



Description. Shell above medium size, subelliptical in outline, broader 

 than long, the greatest width near the mid-length of the shell, the hinge- 

 line much shorter than the greatest width, the cardinal extremities 

 rounded. The dimensions of the type specimen are: length of pedicle 



