from the Gulf of California, etc. 389 
slightly gaping also in front, the anterior ventral opening mostly 
closed by thick calcareous plates, one of which partially overlaps 
the other, somewhat marked by converging ridges: both extremi- 
ties broadly rounded ; disk of the valves divided by a broad sulcus 
barred by the lines of growth, and passing from the beaks to the 
middle of the ventral margin ; posterior portion unsculptured ; an- 
terior portion ornamented with crowded, elevated, undulated con- 
centric laminz, decussated by acute radiating ribs arranged’ some- 
what in pairs ; in front of the beaks the dorsal margin is reflexed, 
not appressed, and then doubled upon itself: cardinal valvule 
missing ; interior yellowish white ; cardinal apophysis long, slender, 
a little expanded and excavated towards extremity, directed ob- 
liquely backwards. An extensive prolongation of the epidermis 
closes the posterior margins and forms a long tube for the protec- 
tion of the siphons. 
ength three inches; breadth one and three fourths inches ; 
height two inches. 
This shell is remarkable for its short ovoid form. It is closely 
allied to P. crispata as to its wide gape and epidermal tube and its 
general sculpture ; but the latter shell does not gape so widely, is 
acute anteriorly, and has not its ventral gape closed by calcareous 
plates. Pholas truncata is more cylindrical, truncate posteriorly 
and acute anteriorly. P. concamerata is much smaller, tapers pos- 
teriorly, slightly gaping, and is prolonged by corneous plates. 
PETRICOLA BULBOSA (Pl XV. Fig. 5.) 
Petricola bulbosa Gould: Proc. B. S. N. H. Nov. 1851, IV. 88. 
T. solida, ovato-cuneata, postice hians, straminea, costis numerosis parvis 
compressis radiantibus ornata ad interstitiis concentric? striatis cibus ad 
lentem anteriorem tumidis ; latere antico subgloboso; latere postico com- 
presso, subtriangulari, ad apicem truncato; margine icon ar ege rra 
livido, cicatricibus ealeareis; cardo dente unico ad valvam dextram, subulato, 
recurvo instructus. 
Shell ovate, solid, inequilateral, yellowish white, having the sur- 
ce ornamented with numerous radiating, compressed, rib-like lines, 
and closely wrinkled between them by the lines of growth ; beaks 
at the anterior third, and tumid, as are the disks against the beaks ; 
anterior part broadly rounded; posterior part subtriangular, the 
apex truncate ; ventral margin minewhat constricted ; interior livid 
