276 EEPORT ON ALBATROSS MOLLUSC A DALL. 



Family LYONSIID^. ? 



Genus LYONSIELLA Sara. 



Lyonsiella radiata Dall. 



Plate vin, Fig. 7. 



Lyonsiella radiata Dall, Bull. Mus. Cotup. Zoiil., xvin, p, 442, June, 1889. 



Shell large, thin, pearly, recalling L. gemma Verrill (=insculpta Jeffr.+ 

 ecostata Segueuza), but very much larger, higher, less rounded anteri- 

 orly, less pointed behind, and more produced and rounded ventrally; 

 hinge simple, undulated, with a rather large, arched ossicle; exterior 

 whitish, with a thin olivaceous epidermis raised over five ribs into rather 

 high distant radiating ridges, to which mud adheres tenaciously; incre- 

 mental lines distinct, silky, sometimes prominent; lunule in the right 

 valve impressed, produced laterally, not marginated; interior pearly, 

 with faint radiating sulci, corresponding to the external ridges ; maxi- 

 mum altitude of shell, 13; longitude, 11; diameter, 8.5 mm . 



Hab. — In Magellan Straits, at U. S. Fish Commission Station 2780, 

 in 369 fathoms, mad; and at Station 2785, off the west coast of Pata- 

 gonia, south latitude 18° 9', in 449 fathoms, mud; temperatures 46°.9 F. 

 in both cases. 



There are a large number of acephalous mollusks, not necessarily 

 nearly related, in which a true brauchial septum exists. In a young 

 Perna, supposed to be P. ephippium L., the iuner 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 radically 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, 

 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 sep- 

 tum. There are, however, no orifices in this or in any of the species 

 with a strictly ctenidial septum corresponding to the septal perfora- 

 tions in Poromya or Cuspidaria. 



In Lyonsia radiata we have a similar state of affairs, except that the 

 anterioi 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, 

 rouuded, ocellus like tubercles on each side of it. A wide space sep- 



