366 bulletin: museum of comparative zoology. 



gon, and which also extends to the Gulf of Panama, but that species, instead of 

 having six or seven anterior radial channels like S. agassizii, has from nine to 

 twelve ; the projections of the periostracura are much longer on the anterior part 

 of the shell than posteriorly, giving a subtriaugular profile (as figured in Proc. 

 U. S. Nat. Mus., 17, plate 25, fig. 1894). 



It is difficult to be dogmatically confident as to specific limits in forms like 

 these when one has only more or less imperfect valves without the soft parts, and 

 especially modifiable by the results of drying. If in future these three species 

 ever become known through abundant material and prove to be merely variations 

 of a single type, it will show a marvellous distribution from Puget Sound, south 

 to the Straits of Magellan. But I have not seen any unmistakable specimens of 

 S. macrodadyla from north of the Island of Chiloe, on the Pacific side. 



Solemya (Petrasma) panamensis Dall, d. sp. 



Shell thin, elongate-oval, the posterior end more pointedly, and the anterior end 

 more bluntly, rounded ; periostracum brown, brilliantly polished, recurved over 

 .the margins of the valves, not produced into long processes, though more or less 

 broken up outside of the margins ; anterior part of the shell radiately marked 

 with eight or nine obscure rays, which are more crowded in front and dorsally ; 

 the middle of the valve with a few sparse rays, the posterior part having six or 

 seven more closely adjacent, followed by a smooth unradiated area behind the beaks 

 and above a line drawn from them to the middle of the posterior end of the valve ; 

 beaks flat, with a lozenge-shaped area of ligament visible behind them ; interior 

 bluish, translucent ; the chondrophore strong, projecting obhquely into the cavity, 

 its front margin prolonged as a narrow, elevated rib very obhquely backward in 

 front of the posterior adductor scar ; muscular impressions rather obscure ; in- 

 terior of the valves faintly radiately striated. Lon. of valve exclusive of the 

 periostracum, 39.0 ; alt. 15.0 ; diameter, 8.0-}-; the beaks in front of the posterior 

 end, 14.0 mm. 



U. S. S. " Albatross," station 2799, Panama Bay, in 29^ fathoms, mud. 

 U. S. N. Mus. 110,678. Also at 2973, off Santa Barbara, California, in 63 

 fathoms, mud, bottom temperature 54° F. 



This is more expanded in front and less sharply truncate in front than S. agas- 

 sizii of the same length, and they may be separated at once by the difference in 

 the hinges. S. valvulus Carpenter is a much smaller species and has no anterior 

 prop to the chondrophore. 



An examination of specimens of Pthonia, Clinopisfha, Dystactella, and Solemya, 

 from the Palaeozoic beds of the west, shows that all the groups except the last 

 have the valves completely closed, the periostracum not extended beyond the 

 valve margins, and the ligament external. Solemya radiata Meek and Worthen, 

 a fine species from the Carboniferous of Illinois, has the ligamrnt external and 

 the hinge apparently very similar to that of S. agassizii. The other characters, 

 however, would hardly allow it to be united in the same subgenus. 



