32 
BULLETIN OF THE LABORATORIES 
crossed by radiating markings, well-defined behind, forming anteriorly 
crowded, obsolescent striae.” 
Our specimens are smaller than those described by Meek, and 
more convex, but otherwise they agree. Length, .36, height, .16. 
This shell is almost identical in outline with M. obsoletus, but is much 
more gibbous. The possibility that it is merely the young of that spe- 
cies cannot be disputed. 
S. Macrodon carbonaria, Cox. 
(Plate IV, Figs. 21 and (?) 14.) 
This is a very pretty and somewhat common species. It is easily 
recognized by the strong radiating ribs which are much more promi- 
nent in the area marked off by the posterior umbonal ridge. The lar- 
gest specimen measures 1.40 long by .625 wide and is truncated ob- 
liquely behind. The concentric markings are plainly seen. 
Figure 14 shows the inside of a valve which, from the inferior 
size and projecting anterior margin, could be identified with M. tenu- 
istriata, M. and W., though, as the shell is but a fragment, it would be 
venturesome. 
Genus Prothyris. 
Prothyris elegans, Metk.) 
(Plate IV, Fig. 3.) 
The specimen on which this identification rests is imperfect and 
the anterior is not fully preserved. The description of the species is 
as follows : “ Shell compressed, elongate oblong, the length being 
about three and a half times the height; ventral and dorsal margins 
straight or a little arched ; the latter with a faintly defined marginal 
furrow, below which there is usually an obscure ridge, also parallel to 
the dorsal margin ; posterior extremity obliquely sub-truncate, the most 
prominent part being below the middle ; beaks compressed, depressed, 
not distinct from the dorsal margin, and placed about one-eighth or 
one-ninth the length of the valves behind the anterior extremity; 
notch of the anterior margin well-defined and extending about one 
half way up to the beaks; ridge from the inner angle of the notch 
narrow, flat, widening slightly from above ; anterior margin above the 
