478 Mr. A. J. Jukes-Browne on the Shells 



bivalves. In 1902, however, he described a second true species 

 of Psephis, which he named P. ovah's, and in 1903 he figured 

 a fossil species from the Pliocene deposits of the Pacific coast 

 of America. 



According to Dr. Dall, the animal presents some peculia- 

 rities, having the mantle-lobes united below, with only an 

 anterior opening for the foot ; the siphons are short and 

 simple, and the embryonic young are retained within the 

 mantle-cavity for some time, so that it is pseudo-viviparous. 



The shell is small, oval, smooth, with a feebly defined 

 lunule and no escutcheon. In the type (P. lordi) there are 

 three cardinal teeth in each valve. The anterior left and 

 the posterior right dorsal margins are narrowly grooved, and 

 the opposite margins are bevelled to form ridges which fit 

 into the grooves ; and there is a distinct angular pallial sinus. 

 Dr. Dall, however, is mistaken in saying that the teeth are 

 entire and that the valve-margins are not crenate. 



When a valve is examined under a strong lens or an inch 

 ol jective it is seen that both the median teeth are grooved, 

 as well as the right posterior. It is also seen that the inner 

 margin is faintly striated tangentially, as in the case of 

 Tran senella, and that it is also minutely crenulated on the 

 anterior and venlral sides. Such a combination of marginal 

 characters is very interesting, and, so far as my experience 

 extends, it is a unique structure. 



I have not been able to examine any other species o£ 

 Psepltidia to see whether they present the same marginal 

 features, but I should certainly expect to find the tangential 

 striation, even if there was no crenulation. 



With regard to the affinities of Psephidia, the shell has 

 much resemblance to that of Gemma, but the animals differ 

 in several important particulars, although they both have the 

 habit of retaining the embryonic young within the mantle- 

 cavity. On account of this protective habit Dr. Dall would 

 unite them into a distinct subfamily ; but I cannot agree 

 with him in regarding it as of more than generic importance. 



If the shell of Psep/ndia is compared with that of Gom- 

 phina, especially of G. undulosa, it will be seen that there is 

 a great resemblance between them, the former only differing 

 in the angular form of the pallial sinus and in the margins 

 being curiously sculptured instead of smooth. The two 

 groups of shells can, in fact, be defined in the same generic 

 terms, with distinct subgeneric differences. At present we 

 do not know anything about the animal of Goniphina, alid it 

 may be found that its mantle-lobes are united below and that 



