596 



MOLLUSCA 



Abranchial l 



3. TAGELUS Gray. Shell elongated with the dorsal and ventral mar- 

 gins nearly parallel; umbo central; 2 cardinals in each valve; foot too 

 large to be retracted into the shell; siphons separated at the end; pallial 

 sinus very large: 2 species on the Atlantic coast. 



T. gibbus (Spengler) (Fig. 940). Shell thick, rounded at both ends; 

 umbo back of the middle, and ligament back of it; length 9 cm.; height 



3 cm.; width 2 cm.: Cape Cod to 

 Texas, common on muddy bottoms 

 in shallow water, in which it bur- 

 rows; Europe 



T. divisus ( Speng. ) . Shell small 

 and delicate > elliptical in shape, 

 ends rounded > the posterior being 

 somewhat more acute than the ante- 

 rior; umbo nearly central; length 35 mm.; height 12 mm.; width 9 mm.; 

 surface smooth, with a band of purple passing from the umbo across the 

 shell: Cape Cod to west coast of Florida; West Indies. 



FAMILY 3. SAXICAVIDAE. 



Shell thick, equivalve, gaping at the ends; hinge toothless or with 



1 or 2 cardinals; ligament external, prominent; pallial line sinuous; 

 pallial sinus present; mantle margins fused, leaving a small opening for 

 the small foot; siphons large and long, united, covered with a thick 

 periostracum : 25 species ; cosmopolitan. 



Key to the genera of Saxicavidae here described: 

 di Umbo nearer the anterior end .............................. 1. SAXICAVA 



a a Umbo nearer the posterior end ............................ 2. CYKTODARIA 



1. SAXICAVA Bellevue. Shell symmetrical when young, with 2 minute 

 teeth in each valve; adult irregular in shape, toothless; surface coarse-; 

 ligament external: 12 species, which bore in soft rock and coral. 



S. arctica (L.) (Fig. 941). Shell very variable in 

 form, more or less elliptical, the right valve projecting 

 partly over the left and generally gaping; length 25 

 mm.; height 15 mm.; width 10 mm.; surface divided by 



2 radiating ridges into 3 equal portions; color white, Soxicnro, arctica 

 periostracum yellowish: nearly cosmopolitan; circurn- 



polar, south to the west coast of Florida and Santa Barbara, Cal. ; very 

 common north of Cape Cod, boring in limestone or retreating in crevices 

 of the rocks, among the roots of seaweed, etc. 



2. CYRTODAEIA Daudin. Shell elliptical, the posterior end the 

 shorter; hinge thick, without teeth; ligament external; animal much too 

 large for the shell : 1 species on the Atlantic coast. 



