290 PROCEEDINGS OF THE ACADEMY OF [1898. 



unequal, slightly rugose outside; posterior tooth of valves ii to vii 

 small, very obliquely directed forward ; teeth of valve viii very 

 short, strongly directed forward and roughened. Sinus wide and 

 square, spongy, the area behind it transversely grooved. Eaves 

 rather wide, " spongy " or porous. 



Girdle sparsely covered with short, rigid, obtuse, glossy spines of 

 a blackish-brown color, or occasionally some are whitish. 



Gills in type specimen, 26 on left, 23 on right side ; not quite 

 reaching the anterior end of the foot. In another there are 23 on 

 the left, 25 on the right side. 



Length 15, breadth 8 mm., or smaller. 



Pacific Grove, in small tide-pools 4 to 8 ft. above mean tide mark. 



This species differs markedly from N. Californica (' Nutt.' Rve.) 

 and N. fluxa (Cpr.) 1 in the shorter valves, lack of radial ribs on the 

 anterior and intermediate valves, and more anterior mucro of the 

 posterior valve, and the single slitting of valves ii to vii. I at first 

 considered it a Middendorffia, but the girdle is like that of Ntittal- 

 lina, not having the peculiarly modified spines of the Mediterranean 

 Middendorffia. It lives with a small form of Traehydermon ( Cyano- 

 plax) Raymondi, which is much more numerous in the tide-pools. 

 From that species it may be known by the spiny girdle. Nearly all 

 the specimens collected are much eroded. 



This species may be considered the type of a new subgenus of 

 Nuttallina, with the following differential characters: Intermediate 

 valves with 1-1 slits; anterior valve not radially ribbed ; posterior 

 valve with the mucro post-median, but not terminal. Other char- 

 acters as in Nuttallina. 



At Mr. Heath's request, it is named in honor of Mr. Thomas, who 

 found the first specimens. 



1 =Ch. scaber Rve., 1847, not of Blainville, 1825. 



