442 BULLETIN OF THE 



question to Pelecypods of a more ordinary type will be a little more clear than 

 they were left by the light of some of Dr. Pelseneer's transcendental hypotheses. 



Before discussing matters of theory, the observed facts will be briefly stated. 



True Gtenidial Septa in Felecypods. — There are a large number of acephalous 

 mollusks, not necessarily nearly related, in which a true branchial septum 

 exists. In a young Perna, supposed to be P. ephippium L., the inner edges of 

 the ctenidia are united to each other their whole length, behind the foot. The 

 outer edges are attached to the mantle or visceral epiderm, so as to form a 

 complete chamber like that of Cuspidaria, but of which the derivation is radi- 

 cally different. In Modiolarca trapesina Lam., from Cape Horn, the ctenidia, 

 from below the anal siphonal orifice to and around the foot, are united as in 

 Perna. The chamber thus constituted is crammed with the young fry at the 

 proper season. In Lyonsia Beana Orb., the united ctenidia are attached above 

 the rudimentary siphonal septum, and extend forward to and around the foot. 

 They are attached to each other and to the mantle or to the ventral surface of 

 the visceral mass by their edges, and form a most complete chamber, a true 

 ctenidial septum. There are, however, no orifices in this or in any of the 

 species wdth a strictly ctenidial septum, corresponding to the septal perforations 

 in Foromya or Cuspidaria. 



In Lyonsiella radiata Dall, a large new species from Patagonia, we have a 

 similar state of affairs, except that the anterior inner edges of the gill are not 

 so closely united, around the foot. The part played by the siphonal septum in 

 this species is insignificant; it is in fact hardly perceptible. The infolding of 

 the mantle edge around the siphon is very wide ; its outer edge is nearly plain ; 

 within this edge a short distance, is an elevated ridge with a single row of 

 small rounded ocellus-like tubercles on each side of it. A wide space separates 

 this range of processes from the margin of the branchial orifice, which is pro- 

 fusely papillose with arborescent papillae. A lunate depression lies between 

 this and the much smaller, plain-edged, nearly linear anal orifice, while in 

 front of it the pedal opening forms a minute narrow slit, with granulated mar- 

 gin. In this form the palps are represented by a slightly raised edge around 

 the mouth, not produced or elongated at the sides. A languette or curtain 

 valve hangs behind the branchial orifice below the narrow septum. 



Tme Siphonal Septa in Felecypods. — A partial siphonal septum is common 

 among Pelecypods, especially short-siphoned forms, where the internal 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 membranous shelf, the posterior extension 

 of the tissues which separate the two siphons, while from near its lateral cor- 

 ners 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 

 different groups of the old genus Cardium. In G. edule a short septum is 

 present, and is figured by Deshayes (Moll. Algerie, pi. xcvii. fig. 6), in which 



