NATURAL SCIENCES OF PHILADELPHIA. 371 



the young of a variety of the above species, or distinct. Should it prove so, I 

 suggest the name P. gambrina. 



Cardita Brug. 



C. monilicost a. Shell nearly circular ; beaks small, submedian, cardi- 

 nal border straight or faintly arcuate. Surface marked by from fourteen to 

 seventeen large rounded ribs, strongly raoniliform; interspaces narrow, acute. 

 Posterior muscular impressions largest. Pallial line broad and distinct but not 

 impressed. Internal margin coarsely crenulate, one large square tooth, cor- 

 responding with each interspace between the ribs ; extreme edge undulated. 

 Hinge robust. 



Length. -19 in.; width, -2 in.; depth of single valve, -05 in. 



From the Tertiary (probably Miocene) of Santa Barbara, Cal. 



This beautiful little shell is somewhat variable in the size and prominence of 

 the nodulations of the ribs, the number of the ribs themselves, and the depth 

 of the valves. One specimen before me, with about the depth given above, is 

 larger and wider, so that the proportions are a little different. It can, how- 

 ever, be readily distinguished by its small size, subcircular form, and the ap- 

 proximate number and beading of the ribs. 



Perna Adanson. 

 Modiola Lam. 



P. Tex an a. Elongated subtriangular. Beaks terminal, small ; umbones 

 prominent, though small ; anterior end rounded, narrow ; cardinal margin 

 straight, basal broadly sinuous; posterior basal angle abruptly rounded; poste- 

 rior ed^e broadly convex, uniting, with a regular curve, with the cardinal line. 

 Umbonal ridge high, convex; anterior slope abrupt and slightly convex near 

 tbe margin towards the beak, between which convexity and the ridge is a 

 slight concavity ; posterior slope regular. Surface covered by numerous, radi- 

 ating, dichotomous ridges, flattened above, separated by deep depressions and 

 crossed by fine lines of growth. These ribs almost disappear on the anterior 

 umbonal slope for about the marginal half of its width. 



Length from beak to posterior angle, 2 inches. Width, at a right angle to 

 the first measurement, *9 in. Depth of valve, -4 in. 



From a coarse brown, highly fossiliferous Eocene sandstone from Caddo 

 Peak, Texas. My collection, from Dr. Moore, State Geologist of Texas. 



Differs from P. Mississippiensis {Modiola id. Con.) in having the cardinal line 

 longer and the shell less oblique. The ribs, which in the latter species entirely 

 disappear in advance of the umbonal ridge, become much smaller in the same 

 part of the present species, but are persistent throughout. All of the ribs are 

 larger and wider apart than in Conrad's species. 



Morrissia Davidson. 



M. Horni i. Shell small, lenticular, flattened. Surface minutely granular, 

 and sometimes with a few indistinct lines of growth, often entirely absent. 

 Outline varying from almost perfectly circular in young shells to indistinctly 

 rounded subquadrate, (exclusive of the beak,) the greatest width being towards 

 the basal margin, which is compressed. Lower valve, beak acute, area narrow 

 and not so long as the width of the shell. Upper valve sometimes marked by a 

 faint, linear, longitudinal depression, running from the foramen, about half the 

 length of the shell. Foramen large, encroaching on both valves, in the usual 

 manner in this genus, but most strongly on the upper valve ; pentagonal in 

 shape, the upper angle (in the lower valve) acute, lower portion more elongated 

 and with tbe two lower angles often rounded. 



Length, -11 in. ; greatest width, -11 in. ; diameter, .015 inch. 



From the Miocene (?) of Santa Barbara, Cal. 



1861.] 



