1894. PEOCEEDINGS OF THE NATIONAL MUSEUM. 703 



be the true answer, whatever the morphological equivalents of the 

 septum may prove to be in any given case. The proposed order " Scj)- 

 tihranchia^^ seems to me founded merely on extreme specialization of 

 organs whicli may be expected to vary almost infinitely and of which 

 the intermediate and connecting stages will probably be found fully 

 exemplified in the various genera of Anatinacea when exhaustively 

 investigated. Toward that desirable state of our knowledge the 

 preceding notes will contribute data of importance. 



Geuus LYONSIELLA, Sara. 



LYONSIELLA ALASKANA, new species. 



Plate XXV, fig. 2. 



Shell thin, large ibr the genns, inequilateral, the anterior end shorter 

 and more vertical, the posterior end mure rounded; covered with a pale, 

 yellow, silky epidermis considerably infolded around the margins of 

 the valves; sculj^ture of fine, distant, radiating, elevated tlireads about 

 half a millimeter apart near the margin; the interspaces crossed by 

 silky lines of growth which are occasionall}^ emphasized as if at rest- 

 ing stages of growth; interior faintly pearly; hinge line edentulous, 

 with a large lithodesma shaped like a flattened shell of Taginella, with 

 a deep sinus in the wider (posterior) end; beaks moderately prominent, 

 much incurved; lunule larger on the right valve, small, heart-shaped, 

 polished; a narrow polished strip on the posterior dorsal edge of the 

 valves may represent an escutcheon. Length of shell, 24; height, 24; 

 diameter, IG mm. 



Station 2859, in 1,509 fathoms green ooze, soathwest from Sitka in the 

 Gulf of Alaska; bottom temperature, 34.9o F. No. 123500, U.S.:N.M, 



This species closely resembles externally L. 7-adiata, Dall,* from the 

 Straits of ^Magellan, but is larger, with the anterior end more vertically 

 truncate, the posterior end and base more evenly rounded, and the beaks 

 more central and inflated. 



For the purpose of comparison with Halicardia the soft parts of this 

 species were examined. In a general way the arrangement of the parts 

 is not unlike that in L. papyraGea, Smith, as figured in the ChaUenger 

 report (Anitomy of molhisks, pi. ii, fig. 8). The anal siphon is short 

 and smooth edged, with a somewhat granular exterior; the branchial 

 siphon is surrounded by a single row of large tentacular papillae, each 

 tentacle being subtriangular, with a projecting barb-like point at each 

 side near the base of insertion, the whole surface distinctly villous and 

 slightly compressed in the same plane as the valves; there are ten of 

 these papillae on each side, diminishing in size anteriorly, with a small 

 one in the median line in front; these and the anal siphon are sur- 

 rounded by an area of nearly bare membrane (which I call the perisi- 

 phoual area) extending to the mantle margin; behind the anal siphon 



*Pioc. U. S. Nat. Mus., xii, p. 276, pi. viii, fig. 7, 1889. 



