752 



LAMELLIBRANCHIATA. 



united, and there are long conjoined siphons, with a single branchia 

 on each side. The shell is in the young state similar to that of 

 Th7'acia, but one or both of the valves ultimately become fused 

 with a secondarily-produced adventitious calcareous tube, of con- 

 siderable size (fig. 638). 



The genus Clavagella itself comprises forms which burrow in 



rocks, corals, &c, and in which 

 the shell is inequivalve, the left 

 valve being fused with a long cal- 

 careous tube (fig. 638), while the 

 right valve lies freely in the in- 

 terior of the tube, the latter being 

 often divided by a longitudinal 

 partition. This singular genus is 

 known to have existed in rocks as 

 old as the Cretaceous, and still 

 survives. In the nearly allied 

 genus Aspergillum (Pliocene and 

 Recent), both the valves of the 

 shell are fused with the calcareous 

 tube, which is closed below by a perforated disc with a minute 

 central fissure. 



Fig. 638. — Clavagella cretacea. Chalk. 



