128 Miscellaneous. 



adherent, but the foot has degenerated to a useless organ, and the 

 form of the body still somewhat resembles Venus, the shell, how- 

 ever, gaping at the aboral or siphonal end. 



In Solen the edges of the shells cannot be brought together, or 

 they gape, as it is said. In this form the new type has become 

 established, and the animal resembles a cylinder ; the large siphon 

 fills up the aboral or gaping portion of the shell, while the boring- 

 foot fills up the oral pole of the shell, the mantle being nearly closed 

 between the foot and the siphonal openings. 



The shells of Mactra are small for the body, and the siphons are 

 so large that they cannot in any way be drawn into the shell ; a 

 large portion of the mantle also is without the limits of the shell, so 

 that the edges of the shell do not touch even in life. 



In Teredo no hinge-teeth are present, nor is even a ligament 

 formed, an organ that is present in all other Lamellibranchs, except 

 the members of this family and the next one to be considered ; 

 besides this a new element is found, namely accessory shell-pieces. 

 The enormously developed siphon is four or five times the size of 

 the rest of the body. The mantle-edges are firmly united except at 

 the oral pole, where the boring-foot protrudes, and at the openings 

 of the siphon. The true shells as well as the accessory pieces are 

 movable, that is, not united with the calcareous secretion of the 

 mantle. 



In GastrocJicena the shells are very small, but still movable, the 

 animal being enclosed in a calcareous shell, the secretion of the 

 siphon. In ClavageUa, a similar form, one shell is welded to the 

 siphon-shell, the right one only being free, and in the extreme form 

 of Aspergillum both shells are immovably fixed in the shelly tube 

 that encloses the animal. 



The freshwater forms Cyclas, Cyjirina, &c. are probably related 

 to Cardium and have received their new forms by moving into fresh 

 water. 



In summing up, Dr. Sharp showed two branches in the Lamelli- 

 branchiata, one going off from a form related to Area, the other 

 toward Ostrea, the fulcrum moving from a position between the two 

 equally large adductors toward the oral pole of the body. This 

 brought the anterior adductor in a line with the fulcrum and poste- 

 rior adductor, where, being of no use, it disappeared. 



In the other direction development is in the antero-posterior 

 direction, the shell, however, not taking part in the growth until a 

 form is reached where the shell is exceedingly small and the animal 

 protected by a supplementary deposit of carbonate of lime. — Froc. 

 Acad. Nat. Sci. Philad. March 6, 1888, pp. 121-124. 



