THE NATURAL HISTORY SURVEY. pals 
(double) teeth in the right valve. They are generally long, 
lamellar and elevated, and either straight or curved. 
Distribution: North and South America, Europe, Oceanica, 
Asia, Africa, New Zealand, East Indies and Madagascar. 
The young of both Spherium and Pisidium are contained in 
the inner gill and are of gigantic size as compared with that of the 
parent, sometimes being one-sixth the length of the adult. When 
born they ‘‘are very active, climbing about submerged plants and 
often suspending themselves by byssal threads.” (Tryon.) The 
genus is very plentifully distributed, the species being very 
numerous in individuals. The group is hermaphroditic. 
KEY TO SPECIES OF SPH/RIUM AND CALYCULINA. 
A. Shell solid, epidermis dark. 
a. Umbones situated anterior of the center of the shell, 
giving it a transversely elongated outline. 
1. Shell trigonal, umbones elevated and inflated, 
almostvsnvooth jy vatac a conetersite Gia c etateserntsy tate atetan solidulum 
2. Shell oval, umbones not as much elevated as 1, 
very nll; coarsely writen) .c fe sc cd s.asig. 0s ceo stamineum 
3. Shell transyersely elongated, umbones small, de- 
Pressed almost SMOOEM ss). se acl ele viele te alee) striatinunt 
b. Umbones situated near the center of the shell, depressed; 
shellitransverselynovale ayer «ccs evclecte cts six. ash gare dee ie simile 
B. Shell fragile, epidermis light. 
a. Shell transverse. 
1. Shell very transverse, umbones elevated and 
placed well toward the anterior end......... transversuni 
2. Shell transversely oval, umbones depressed and 
placed centrally tices ate ata ae eee fabale 
b. Shell rhombic. 
1. Shell rhombic-ovate, posterior end squarely 
truncated, umbones placed near the center..../runcatum 
2. Shell muchinflated, rhombic-orbicular, posterior 
margin truncated, very high compared with 
the anterior margin, beaks a trifle anterior, 
SAMS eae ae th ltaker aha Ne is avalava biesiale asi h aPee securum 
c. Shell oval. 
1. Shell somewhat inflated, small, ends rounded... occidentale 
2. Shell compressed, large, posterior end trun- 
PRM ae, ere Or Ae Cis Aicla-v'a brewed fie ow h partumeium 
37. Sphzrium solidulum Prime, pl. xxvii., fig. 4. 
Cyclas solidula Prime, Proc. Bost. Soc. H. N., Vol. IV., p. 158, 1851. 
Cyclas distorta Prime, |. c., p. 158, 1851. 
Shell: Of good size, inflated, very solid, inequilateral, tri- 
gonal; umbones inflated, somewhat elevated, placed anterior to 
