24 THE VOYAGE OF H.M.S. CHALLENGER. 



The two pallial posterior apertures are prolonged by two siphons joined along their 

 whole length, except towards the distal extremity, where they are slightly separated 

 (PI. IV. fig. 6, s, t) ; at this point they are surrounded by a common sheath (?i;). 

 The extremity of the branchial siphon bears, ventrally, two pairs of claviform tentacles ; 

 the anal siphon has only three of these organs. The base of the branchial siphon is 

 closed by a partition, in which is a linear aperture {h) of small extent. 



The pallial cavity is divided into a dorsal and a ventral chamber (PL IV. fig. 6, h, i) 

 by a muscular partition (<?), extending from one adductor to the other, and joined to 

 the mantle on each side by a more delicate membrane (PL IV. fig. 3, h). The foot 

 issues near the middle of this partition, which has been considered as the body-wall. 



The foot {d), which is rather delicate and linguiform, has a byssal groove on its 

 posterior surface. Its posterior retractor muscle (PL IV. fig. 6, n') is very delicate, 

 single, and only bifurcated near its insertion on the shell, as in the two preceding 

 genera {Poromya and Silenia). 



The mouth (a) is rather widely open. According to Dall,^ it has no labial palps. 

 In reality it has two pairs, but these organs are much reduced, and would scarcely be 

 visible in badly preserved specimens (PL IV. fig. 5). The anterior pair rest on the 

 anterior adductor muscle ; the posterior pair are carried far backwards on each side of 

 the foot, as in Solen. In Cusindaria fragilissima and Ctisjndaria curta even the 

 anterior pair are very slightly developed (PL IV. fig. 2). 



The muscular partition (e), of which we have spoken above, is attached to the shell 

 by two large muscular bundles in front of the posterior adductor, and by two others 

 behind the anterior adductor, so that, seen from the side (PL IV. figs. 1, 4), the parti- 

 tion seems to form a curved mass, which Jeff"reys,^ looking through the transparent 

 mantle, took for " pink gills." 



The visceral mass lies in the chamber dorsal to the partition, and fills the greater 

 part of it. Between the visceral mass (/) and the posterior retractor muscle of the 

 foot {n') is a delicate sagittal partition {p). On the wall of the visceral mass there is 

 no trace of gills any more than on the muscular partition. This partition is absolutely 

 homologous with the " branchial " partition in Poromya and Silenia, and consequently 

 represents the gills of Pelecypoda. 



Instead of the three groups of apertures observed in Silenia (PL III. figs. 8, 9), we 

 only find here four separate apertures, arranged in the same manner in all the species 

 (PL IV. figs. 1-4), and situated near the median line. Except for these apertures, and 

 the little Hps which border them, the whole surface of the partition is uniform. In 

 Cuspidaria fragilissima (PL IV. fig. 1) and Cnspidaria curta (PL IV. fig. 2) there are 

 two pairs of apertures behind the foot, while in Cnspidaria rostrata (PL IV. figs. 3, 4) 

 and Cuspidaria platensis there is only one. 



^ Nature, vol. xxxiv. p. 122. - British Conchology, vol. iii. p. 45. 



