364 



SHELLS AND SHELL-FISH. 



shell free^ oblong-ovate, thin, with an internal com- 

 pressed tooth in each valve and accessary pieces. 



Pholas Linn. Shell gaping at both extremities over the 

 umbones ; ligament none. {^fig. 122. f.) 

 P. dactylus. Sow. Gen. f. 1. 



Pholidcea Leach. Resembling Pholas, but the anterior 

 end is closed by a thin calcareous prolongation of each 

 valve, and the posterior end has a cup-shaped ap- 

 pendage, {fig, 122. g.) 



T. papyracea. Sow. Gen. f. 3. Man. f. 56. 



Martesia Leach. General characters of Pholas, but 

 both extremities are closed. 



M. clavata Auct. {fig. 122. i.) 



Xylophaga Sow. Habit of Pholas ; but the shell is 

 orbicular, widely gaping anteriorly, and with ac- 

 cessary valves ; each valve with an incurved tooth, 

 and an internal transverse rib. 



dorsalis. Sow. Man. f. 50, 51. 



Teredo Linn. Shell tubular, irregular ; the valves 

 pholas- shaped, short and nearly orbicular. 



Teredo Linn. Valves resembling Xylophaga; placed 

 at the thickest extremity of a long, irregular tube, 

 " which is open at both ends, the anterior end di- 

 vided into two apertures furnished with two oper- 

 cula."* {fig. 114. A:,/.) 



T. navalis. Sow. Gen. Man. f. 48, 49. 



Teredina. Tube clavate, irregular, nearly divided into 

 two, by a projection on each side within, thus form- 

 ing a double opening at the smallest extremity; 

 valves as in Teredo, but wholly external, incrustated 

 with the tube, and having accessary valves. 

 T. personata. Sow. Gen. Man. f. 46, 47. (fig. 114. i.) 



* I have not leisure to look into these characters, and have therefore 

 adopted Mr. Sowerby's account of this and the next genus. 



