16 



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 laminse, 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 valvulsB 

 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. 



Length 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 (PI. XV. Fig. 5.) 

 Petricola lulbosa Gould: Proc. B. S. N. H. Nov. 1851, IV. 88. 



T. solida, ovato-cuneata, postice hians, straminea, costis numerosis parvis 

 compressis radian tibus ornata ad interstitiis concentrice striatis : apicibus ad 

 trientem anteriorem tumid is ; latere antico subgloboso ; latere postico com- 

 presso, subtriangulari, ad apicem truncato; margine ventrali sinuato : intus 

 livido, cicatricibus calcareis ; cardo dente unico ad valvarn dextram, subulate, 

 recurvo instructus. 



Shell ovate, solid, inequilateral, yellowish white, having the sur- 

 face 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 somewhat constricted ; interior livid 



