217 



RHYNCHONELLA ? RETICULATA (Hall.) 



Shell varying from elongate triangular to transversely el- 

 liptical, gibbous in the upper part, more attenuate in 

 front, cardinal slopes flattened in most specimens, some- 

 times rounded. Ventral beak small, acute, flattened on 

 the back and closely incurved ; the front half of the 

 ventral valve marked by a broad, shallow, sometimes un- 

 defined sinus. Dorsal valve usually more ventricose, 

 beak obtusely pointed and strongly incurved, a low, 

 broad, scarcely defined mesial elevation marks the centre, 

 and often occupies nearly one third the entire width of 

 the valve : often this fold is obsolete, leaving the valve 

 regularly arcuate from side to side, while in other speci- 

 mens the borders are depressed and the shell flattened in 

 front. 



Surface of the shell, except a small space on the umbo 

 of the ventral valve, covered by a finely reticulate marking, 

 with elongate, generally hexagonal pits or openings, with 

 thin and sharp ridges between. These markings vary in 

 different specimens, and also on different parts of the same 

 individual; being generally finest on the cardinal slopes. 

 The small triangular space near the ventral beak which is 

 destitute of marking has the appearance of having been 

 exfoliated ; but since this is an invariable character in 

 the collection of about fifty individuals, usually varying 

 in size with the size of the shell, it is probably dependent 

 upon a peculiar organism of the animal. 



This species resembles R ? (Atry-pa) coralifera (Pal. N. Y , 

 vol. n, p. 281); but differs in being much broader and less 

 ventricose, and also in the greater breadth and less promi- 

 nence of the mesial fold. 



