i _ in all their movements to Gasteropods. 
350 A Monograph of the Species of Pisidium. 
its utmost extent, it has much the appearance of the foot of 
the Lucinas. The animals make use of it in much the same 
manner, as do the Cyclas, either to dig a pit and bury them- 
selves, or else to crawl on solid bodies." Des Hayes. 
With regard to the shell, the principal difference consists 
in the position of the beaks. In Pisidium, the beaks are 
terminal, that is, the distance from the beaks to the anterior 
margin, is greater than the distance from the beaks to the 
posterior margin. In Cyclas, on the contrary, the position of 
the beaks is generally more or less central, dividing the hinge 
margin into equal portions. Considering the hinge, we find 
the characters very much the same, but the position of the 
cardinal teeth in Pisidium, like that of the beaks, is terminal 
and in Cyclas central. 
The habits and modes of living of the animals of these two 
genera are very much the same. They seem to live either 
buried an inch or two below the water in the mud, or else 
attached to roots and leaves of aquatic plants. 
Their mode of breeding is also the same, both ejecting the 
young from them, when sufficiently mature, which up to that 
time they carry between the folds of the gills. Pfeiffer sup- 
poses the Pisidium to breed by means of eggs, but I have 
myself found the young in the shells of Pisidium, in P. varia- 
bile, P. Kurtzii, for instance. 
The most appropriate time of the year for collecting Pisi- 
dium, seems to be from the middle of April to the first of 
-= July, the breeding season; some species, however, such as 
the P. variabile, I have found at all seasons of the year, the 
winter inclusive, and others, such as the P. ventricosum, are 
seldom found but in the early summer. | Live specimens, for 
examination, may easily be preserved for some time in jars or 
. vases, and, on the occasional application of fresh water, they 
exhibit great activity, extending their syphonal tube and foot; 
not only in mounting the sides of the vase, but also in crawl- 
ing on the under surface of the water. deg are very similar | 
