11 



a narrow double rounded rib, the surface of which is triangularly striate; then 

 by a depressed space equally wide, with its sides accurately defined, and trans- 

 versely sculptured. The whole surface of the body and auricle posteriorly is 

 unmarked, except by growth lines, and the depression of surface at the com- 

 mencement of the latter. 



Pallet large ; the jointed portion about the same length as the stalk, rather 

 wide, with lateral extremities acuminate and inclining upwards. Sides of the 

 joints fringed. These joints number about sixteen or eighteen. Stalk very 

 slender and rounded. 



Dimensions. Length and breadth of valves about inch. Length of pallets 

 1} inch. Breadth of upper portion of pallets about inch. 



Bab. Bay of San Francisco, Cal. Rev. J. Rowell. My cabinet and cabinet 

 of Mr. Rowell. 



Observations. The valves of this species bear a very strong resemblance in 

 size and form to T. megotara, Hanley. From X. bipennata, Turton, it is dis- 

 tinguished by the basal margins of the auricle and anterior area not being on 

 the same transverse line, and by its broader, more closely jointed and shorter 

 pallets. 



From X. cucullata, Norman, it is distinguished by its different surface mark- 

 ings and the sharp angle of the anterior area with the body. 



X. fimbriata has a shorter pallet and much smaller and differently formed 

 valve ; the same may be said of X. minima. X. Stutchburyi has very fine striae 

 on the anterior area, and the pallets also differ. The latter, in the present 

 species, are somewhat like those of X. palmulata, Lam., but are longer than in 

 that species, and differ in the relative length and diameter of the stalks, and 

 also by their fringed joints. 



Our fellow member, Dr. A. L. Heermann, informed me some time since, of the 

 existence of a shipworm at San Francisco, which, at the period of his visit there, 

 (several years ago,) was committing great ravages in the destruction of the 

 wharves of that city. It appears since to have become rare, and it was only 

 after diligent inquiry that my friend, Mr. Gabb, found specimens in the collec- 

 tion of Rev. J. Rowell, who kindly sent me one. 



Note on Diplothyra. 



In Am. Journ. Science and Arts, May, 1863, p. 455, is a note by Dr. Stimp- 

 son on my genus Diplothyra, in which he asserts its identity with Martesia, upon 

 the ground that the additional dorsal valve is not a generic character, being 

 sometimes developed by Martesia cuneiformis. Although many of the speci- 

 mens of Diplothyra Smilhii do not exhibit the double dorsal valve, I attribute 

 it to the suppression or abortion of that appendage, and regard the double valve 

 as the normal condition of the genus and species. The shells which passess it 

 are generally the largest and best examples, and its non-appearance in others 

 is not surprising when we consider the character of the substance in which 

 they reside, and the evident compression and distortion of many of the speci- 

 mens. The species of the family Pholadidse are very liable to distortion and to 

 the suppression or division of their dorsal plates ; for instance, in Dactylina 

 dactylus they are frequently not developed at all ; in Martesia striata several 

 specimens before me exhibit incipient stages of a division of the umbonal plate 

 into three, but I certainly would not on this account doubt the generic distinct- 

 ness of Penitella, which is characterized by having three valves. 



If Dr. Stimpson's'-lf. cuneiformis, when perfect specimens are selected, exhibit 

 dorsal valves corresponding with my figure of D. Smithii, then either M. cunei- 

 formis must be a Diplothyra, or else the specimens alluded to are not veritable 

 cuneiformis. I have very closely examined many specimens of cuneiformis, striata, 

 obtecta, etc., but have not detected a second proper mmbonal valve upon them. 

 Mr. S. P. Woodward, some years since, regarded the differences in the urn- 



