266 REPORT ON ALBATROSS MOLLUSCA DALL. 



mon among Pelecypods, especially short-siphoned forms, where the in- 

 ternal septum may, to a certain extent, make up for the absence of the 

 long and complete division between the passages in those forms with 

 long siphons. The septum is usually a mere subtriangular thin mem- 

 branous shelf, the posterior extension of the tissues which separate the 

 two siphous, while from near its lateral corners radiate the muscles 

 which in those forms with a pallial sinus serve to retract the siphons. 

 Below it is the more fleshy languette or curtain-valve which closes the 

 incurrent siphonal opening when required. Among those forms in 

 which we may find the septum especially well developed are the dif- 

 ferent groups of Cardium. In C. edule a short septum is present, and is 

 figured by Deshayes (Moll. Algerie, PI. xcvu., Fig. 6), in which an open- 

 ing appears above and behind the valvular languette. I suspect this 

 to be due to lesion, as I have not found such an opening iu any of the 

 species of Cardium I have examined. In another species, C. Mans (op. 

 cit., PI. xcvi., Fig. 2), the septum is considerably extended forward. The 

 present species has the ordinary gills of Cardium well developed, with 

 their posterior anchorage high up and near the siphonal septum at its 

 origin. The septum thin, slightly fibrous, but nowhere fleshy, ex- 

 tends forward to the foot and on each side of it. In this case there is 

 no orifice above the languette, or elsewhere in the partition. Doubtless 

 an exhaustive search would find many other groups in which certain 

 members exhibit a siphonal septum, more or less completely dividing 

 the peripedal chamber. Until the character has become more particu- 

 larly specialized and permanently established, it is evident it can have 

 but a minor value as a guide to the systematist, or a test of his classi- 

 fication. 



Suborder ISOCARDIACEA ? 

 Family IiSOCARDIID.F. 



Genus CALLOCARDIA A. Adams. 



Callocardia A. Adams, Ann. Mag. Nat. Hist., xm, p. 807, 1864. Ball Bull. Mus. Comp. 

 Zool., xn, p. 272, 1886. Type C. guttata A. Adams. 



To the kindness of Mr. Edgar A. Smith, of the British Museum, I 

 owe a careful drawing of the interior and hinge of the unique left valve 

 of Adams' type of Callocardia. This is reproduced here, having never 

 been figured. MeiocardiaH. & A. Adams differs from Callocardia not 

 only in the shape of the shell but in the form of the cardinals, the 

 principal lamella of which is externally cariuated, the carina running 

 down and outward below and coiling into the spiral umbo. The teeth 

 in the whole of this family are peculiar, they seem appressed against 

 rather than to rise from the cardinal margin and are separated by deep 

 sockets or pointed holes ; the teeth themselves seem to be composed of 

 one or two lamella, springing from the umbonal hollow and termi- 



