METAZOA MOLLUSCA. 199 



rior. 1 A glossy horny layer covers the opening between 

 the valves; when the animal is young (No. 371 a), this 

 horny layer is entire or simply pinked at the anterior and 

 posterior ends, but as the animal grows older it is slit 

 into strips (No. 371). The hinge is without teeth. The 

 internal portion of the ligament is back of the beaks in a 

 triangular receptacle and is strengthened by limy, arched 

 supports. Just in front of the latter are the distinct ante- 

 rior muscle marks. The shell of most primitive forms is 

 nacreous, that is, pearly, but Solenomya is only slightly 

 so. The gills of this genus are in a primitive condition, 

 similar to those of Nucula (see p. 202). The foot (No. 

 370) does much hard work enabling the animal to swim 

 and to bore, so that it is large in proportion to the size of 

 the animal. The mantle is not drawn out into tubes or 

 siphons. 



Anatina truncata Lam. (No. 372), is a delicate, trans- 

 lucent, and pearly shell with the posterior end truncated 

 and the anterior rounded. The two valves are open 

 nearly all the way round, so that, as compared with many 

 bivalves, they afford slight protection to the soft body 

 within. According to Smith 2 one species of this genus, 

 Anatina elliptica, shows the two ends nearly alike, while 

 others have the anterior portion longer than the posterior, 

 the reverse being the case with the young. 



The hinge has a socket for the internal ligament called a 

 fossette, which is strengthened in its position by two limy 

 supports that radiate downward towards the center of the 

 shell (No. 372, specimen at right). 



This genus has only two gills, one on each side of the so 



1 When we speak of the anterior and posterior end of a shell, the 

 latter is mounted with the anterior end away from the observer 

 a favorable position for comparison w^ith one's own body. In other 

 cases the shells are mounted to show certain important features. 

 In a few cases delicate shells have been left as first mounted, owing 

 to the danger incurred in remounting. 



2 Chall. Rep., Zool., XIII, part 35, 1885, p. 77. 



