372 Div. 2. MOLLUSCA.— ACEPHALES. Class 4. 



agape at both extremities. They hve almost uniformly buried in sand or mud, in rocks or in 



wood. 



The Myad^ {Mya, Linn.) — 



Are bivalved shells with a variable hinge. The double tube forms a fleshy cylinder ; the foot is com- 

 pressed. From variations in the hinge MM. Daudin, Lamarck, &c.. have established the following 

 subdivisions, the first three having an internal ligament. 



Lutraria, Lara. — The ligament, like that of the Mactra, is iiiserted in a large triangular fossa in each valve, and 

 in front of that fossa is a small tooth en chevron, but there are no lateral teeth. The gape of the valves is wide, 

 particularly at the posterior end, whence the large double tube for respiration and excremential matters protrudes. 

 The foot, which issues at the opposite end, is small and compressed. The species burrow in sand at the mouth of 

 rivers. 



Mya, Lara., has in one valve a broad, spoon-shaped tooth, which projects into the other valve, in which there 

 is a fossa, and the ligament is stretched from the fossa to the tooth. The species ou our shores burrow in sand. 

 Near to the Myae we ought to place the Anatirne, Lam., that have a small moveable testaceous appendage, connected 

 with the ligament immediately before the hinder teeth. In the Solemya, Lam., the 

 ligament appears externally, but a portion of it remains attached to a spoon-shaped 

 tooth in each valve. There is no other tooth in the hinge. A thick epidermis overlaps 

 the margins of the shell. An example (Te/^/na togata, Poll) lives in the Mediterranean, 

 rxhe animal is so remarkable that it may become the type of a distinct family, for, 

 instead of four lamellar branchiae, it has two only, which are pectinate, or rather pen- 

 nate.] Glycymeris, Lam. (Crytodairia, Daud.), has neither teeth, nor laminae, nor 

 fossa, in the hinge, but a simple callosity, behind which there is an external ligament. 

 The animal is similar to Mya. The best known species (Mya siliqua, Linn.), comes 

 from the Arctic seas. Panopea, Mesnard, Lagr., have in front of the callosity of the 

 Fig. vyi.—AixnuttA -^uhroBtrai* preceding, a strong tooth immediately under the beak, which crosses with a similar tooth 

 of the opposite valve, — a character which affines them to Solen. There is a large species from the hills at the foot 

 of the Apenidnes, so well preserved that it has been sometimes believed to have been brought from the sea. Per- 

 haps we ought to remove from the genus another fossil species, which is almost completely closed at the anterior end. 

 We may arrange at the end of these ditfereut modifications of the Myadse, the Pandora, Brug., which has one 

 valve much flatter than the other, an internal ligament placed crosswise, accompanied with a projecting tooth of 

 the flat valve. The posterior side of the shell is elongated. The animal is more completely contained within the 

 shell than it is in the preceding genera, and the valves close better, but its habits are the same. One native species 

 (Tellina iiuequivalvis, Chemn.), is well known. 



Here, also, we group together some small but singular genera. The Byssomia, Cuv., characterized by an oblong 

 toothless shell, with the opening for the foot very nearly in the centre of the valves, and opposite the beaks. They 

 perforate rocks and corals. One species, furnished with a byssus (MytHus pholadis. Mull.), is very numerous in 

 the seas of the north, lliatella, Daud., has a shell that gapes in the middle where the foot protrudes, as in the pre- 

 ceding, but the tooth of the hinge is more distinct. The shell is often armed backwards with [two] rows of spines. 

 The species live in sand and amid zoophytes, &c. The northern seas possess a small species.* 



The Solenes {Solen, Linn.) — 

 Have an oblong or elongated bivalved shell, but their hinge is always furnished with distinct teeth, and 

 their Ugament is always external. 



Solen, Cuv., or Razor-fith, has a shell in the form of an elongated cylinder, with two or three teeth in each valve 

 towards the anterior extremity, where the foot passes out. This is of a conical shape, and is used by the animal 

 to form its burrow in the sand, in which it sinks rapidly on the approach of danger. Several species inhabit our 

 shores. The species in which the teeth approach near the centre of the shell may be distinguished generically. 

 The shell in some of them is still long and straight ; in others it is wider and shorter, and the foot of these is very 

 large. Some such are found in the Mediterranean. In the Sanguinolaria, Lam., the hinge is very nearly the same 

 as in the broad Solenes, and there are two hinge teeth at the middle of each valve ; but the valves approximate 

 much closer at their ends, where they only gape to a slight extent, as in some of the Mactra; : S. rosea is the type. 

 Psavimobia, Lam., ditfers from Sanguinolaria in having a single tooth in one valve, which clasps in between two of 

 the opposite ones. And the Psatnmothea, Lam., have only one tooth in each valve, but otherwise resemble 

 Psammobia. [The Glauconome, Gray, is a genus of the family Solenaceae, " inhabiting some of the great rivers of 

 the continent of China." Tlic shell is thin, oblong, with close margins, and three teeth in each valve. iSolenella, 

 Sowerby, is an interesting genus, partaking of the characters of Nucula and Solen, so that it may be regarded as 

 the link that connects the two families Solenacea: and Mactracea;. "It belongs to the Solenacea;, having the external 

 ligament and the large sinus in the muscular impression of the mantle ; but resembles Nucula in having the lateral 

 teeth divided into a series of minute and pointed teeth, diflering from it, however, in not having an internal 

 ligament." The species are South American.] 



The Pholades {Pholas, Linn.), — 

 Have two principal valves, wide and ventricose on the side of the mouth, narrowed and elongated on 

 the opposite side, and leaving at each end a large oblique opening ; the hinge has, like that of the Mya, 



* ^BvssouU, IliattUa, Biopholius, and Pholeobiui of Leach, are aU reduced to the Saxicavaof Lam., by Sowerbv, and not unrensonnbly 'i 



