some new and peculiar Mollusca. 425 



both was the same, viz. the " compass-scallop," or Ostrea 

 pleuronectes of Liiin^. Swainson called it Pleuronectia : his 

 description, however, is both inadequate and incorrect ; and 

 that name had been appropriated by Rafinesque to a well- 

 known family of fishes. The descriptions of Amusstum and 

 Amusium given by Messrs, Adams and Dr. Woodward are 

 also unsatisfactory. The peculiar character of the genus 

 consists in each valve being fluted inside, without any corre- 

 sponding ribs appearing outside as in certain species of Pecfe?i. 

 The use of these internal ribs may be to strengthen and sup- 

 port the delicate fabric of the shell. 



Amussium lucidum^ ^ Jeffr. 



Pleuronectia hicida, Jeftr, iu Wy ville Thomsou's ' Depths of the Sea,' 

 p. 464, f. 78. 



Shell roundish, with a tendency to become longer and oval 

 in some specimens, equilateral, flattened, very thm, semitrans- 

 parent, and glossy : scidpture^ in the upper valve none, except 

 fine concentric and close-set lines of growth, which are more 

 conspicuous towards the outer edge in front ; the lower valve, 

 which is much smaller than the other, is marked with regular 

 but crowded concentric strise : colour white : margins semi- 

 circular in front and at the sides, whence there is an abrupt 

 slope to the back : heaks minute and inconspicuous : ears 

 small, nearly equal in size, right-angled : liinge-line straight, 

 occasionally spinous : cartilage-pit minute, triangular, with a 

 curved base : hinge-plate broad and smooth : inside pearly, 

 furnished with 9 slight and thread-like ribs, which are visible 

 on the outside, owing to the thinness and semitransparency 

 of the shell ; these ribs occupy only the middle of the interior, 

 and do not extend to the umbonal or posterior area, nor in 

 full-grown specimens to the front; each rib terminates ab- 

 ruptly in a rounded knob or point ; one is in the centre, and 

 the others diverge on either side, the last or hindmost pair 

 forming the base of each ear : muscular scars roundish-oval, 

 situate below the beaks. L. 0'5. B. 0*5. 



Station 12, 1450 fms., lat. 56° 11' N., long. 37°41'W., 

 Olohigerina-ooze, and stones. ' Porcupine ' Exp., 1869, 557- 

 862 fms. ; 1870, 500-1095 fms. Gulf of Mexico, oflT Alli- 

 gator Bank, 156 fms. (Pourtales) . 'Challenger' Exp., off the 

 Azores, 1000 fms. 



Single valves from all depths are not unfrequently marked 

 in the same way as I have noticed as to Pecten fragilis. 



* Shining'. 

 Ann. &Mag. N. Hist. Ser. 4. Vol. xviii. 29 



