22 THE NAUTILUS. 
species. It has been known to conchologists generally as Cyclas 
elegans C. B. Adams, but Mr. Prime, after lon g and careful investiga- 
tion, finds it to answer every requirement of Say’s rhomboidea, and 
he is now followed by nearly all authorities in calling it by the 
above name. 
S. rhomboideum is a much smaller, though stouter, shell than 
partumeium ; tumid subglobular; beaks not prominent; both ends 
of the shell truncate, so as to give it a rhomboidal appearance ; in- 
terior of valves bluish, exterior olive-green and straw-colored mar- 
gins; hinge strong; cardinal teeth rudimentary, lateral teeth large 
and strong; surface elegantly marked with concentric ridges. 
Length, 4, height, 7-20, breadth, 11-40 inch. 
It inhabits New England and Canada along the northern tier of 
States to Michigan. It is extremely local in habitat, and up to 
1851 was considered a very rare shell. At this time Mr. Whitte- 
more found it quite abundantly near Cambridge, Mass. In the 
depression near Hammond’s Pond, in Pawtucket, where the Planorbis 
jenksii was discovered, can be seen in Spring millions of rhomboideum, 
adhering to stems of aquatic plants and on the dead leaves, while 
under the leaves at the bottom of the pool, in mud, are obtained the 
largest and most perfect specimens of partumeium I have ever seen. 
172.—Spherium securis Prime. 
Shell small, rhomboidal, acutely rounded in front ; posterior mar- 
gin abrupt, forming an obtuse angle with the hinge margin; beaks 
elevated, large, nearly central; approximate at the apex; valves 
thin ; strize very delicate, hardly perceptible without a lens; epider- 
mis variable in color, from greenish-horn to brilliant yellow or 
straw; cardinal teeth very small; lateral teeth elongated. Length, 
37-100, height, 31-100, breadth, } inch. Animal pink, with siphons 
of the same color, causing the shells to appear pinkish while the 
animal is inclosed within them. 
Inhabits the same range of country as rhomboideum. The only 
locality in Rhode Island that I have seen is just below Pontiac in 
the Pawtucket River, under stones at the edge of the water. Some 
years ago I described a shell found in Tyonge Reservoir, under 
the name of Spherium deformis, a shell somewhat larger than typical 
specimens of securis, with a brownish epidermis, anda distorted 
twist in each valve; every specimen found ‘in this place was de- 
formed precisely alike, but on submitting them to Mr. Prime, he 
pronounced them to be only a local variety of securis. 
