1888.] NATURAL SCIENCES OF PHILADELPHIA. 123 



farthest from the fulcrum was always the larger, in fact it must of 

 necessity be so, as more power was needed at this point, while the 

 near one, from tlie fact that it does not require much power, dimin- 

 ishes in size. In Pinna, one muscle is very much, in fact four or 

 five times, larger than the other ; the smaller being close to the apex 

 of the shell, in other words, close to the fulcrum. 



As the fulcrum passes still farther forward, a point is soon reached 

 wdien both muscles come in line with the fulcrum, the larger one in 

 this case takes all the work from the smaller one, which from its now 

 useless position degenerates to disappearance. 



A proceedure from regular to irregular shell is to be seen in the 

 fresh water forms. Unio, he held, is probably a fresh water Mijtilis^ 

 which does not have any byssus present in the adult, but has one in 

 the embryo. A form that closely resembles the oyster can be trac- 

 ed through Aetheria to Muellerla, the so-called fresh-water oyster. 

 The later has both adductors ill the embryo, but only one, like 

 Ostrea, in the adult. 



In passing now in the other direction, Dr. Sharp pointed out the 

 stages connecting the central type to the extreme in AspergUlum. 



In passing out from the central type, the Areas, the group known 

 as the Syphonata appear, where besides the large foot, it is found 

 that the aboral portion of the mantle has united at two or three 

 points, forming one or two tubes. In some forms of Lucina, by the 

 union of the mantle a single tube is formed, the so-called anal siphon, 

 Avhich corresponds to the superior one when two are present ; through 

 this passes the water outwards, the inflowing water passing in through 

 the large space between the mantle edges, as in the asiphonated 

 forms. In this form of Lucina, specialization has only determined 

 the direction of the out-flowing current, which carries off the deoxy- 

 genated water and the excreta. 



In Cardium the siphon is made up of two tubes; in other words, 

 the ingoing and outgoing currents are now determined. The edges 

 of the mantle commence to adhere, leaving room only for the pro- 

 trusion of the foot. In Venus the arrangement is practically the 

 same: — a well developed siphon, large wedge-like foot, which is a 

 locomotor organ, a shell entirely covering the animal when it is 

 closed and two well developed adductors, equal in size. The speciali- 

 zation in this line of development is in the direction of the siphon 

 and closure of the mantle. ATija would represent a form, leading to 

 Solen, here the siphon is large, the mantle more or less adherent, 

 but the foot has degenerated to a useless organ and the form of the 

 body still some what resembles Vemis, the shell, however, gaj)ing 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 neAv type has become es- 

 tablished, 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. 



