The prosobranchs are further subdivided into suborders according to certain peculiarities of the heart and breathing-organs. There is a group of these prosobranchs which gives evidence of an inferior degree of that visceral torsion which is always found in the gasteropods. In this group, or suborder, the heart has two auricles, and there is a pair of gills instead of only a single one. Other internal organs are paired just as they were represented to be in the schematic mollusk. This group also seems to show its primitive character in the want of a proboscis and a siphon, or, in some families, by having the ventricle of the heart traversed by the intestinal canal, just as in the lower class of mollusks, which includes the clams and oysters. For the most part the shells of this group are not typically spiral, but are patelliform, shield-like coverings, with only a suggestion of a spiral form at the very tip of the apex. This group of primitive prosobranchs is included in the following suborder: SUBORDER ~DIATOCARDIA~ This suborder is named from the presence of two auricles in the heart. FAMILY ~ACMÆIDÆ~ The first family to be noted is the /Acmæidæ/. Its principal genus, /Acmæa/, is well represented on both the east and the west coast of the United States. GENUS ^Acmæa^ ^A. testudinalis.^ This species is found in vast numbers all along the New England shore, clinging to the rocks between tides. They are usually called limpets. The shell is solid, conical, with an oval outline, and with no trace of a spiral form in the adult. When the shell is thoroughly cleaned, it generally presents a mottled coloration of pale green, brown, and white. Inside it is white and nacreous, with a large brown [pg357] area under the apex. The animal has a powerful foot, by means of which it is enabled to cling to a rock with great strength. In order to dislodge it, it is necessary to approach carefully, and, taking the creature unawares, quickly slip a broad knife-blade under the foot, otherwise it is quite impossible to tear it from its resting-place. /A. testudinalis/ lives entirely between tides, and ranges from the northernmost waters to New York. It feeds upon algæ and is a very sluggish animal. It has been said to leave its resting-place and wander about in search of food, returning to its original and usual spot when the tide begins to ebb. (Page 343.) ^A. testudinalis,^ variety ^alveus.^ This variety is smaller, more fragile, and oblong. It lives upon eel-grass, its oblong shell being adapted to the narrow leaves of the grass. The coloration is brighter—reddish-brown spots on a white surface. It is exceedingly common on the New England coast. [Illustration: /Acmæa testudinalis/, var. /alveus./]