MACTRA 



the shape of the letter V, the point of which is nearest 

 to the umbo and the branches diverging from it ; next 

 to this on the posterior side, and very close to it is 

 generally placed a very thin sharp tooth; and some- 

 times in one valve the branches of the angularly folded 

 tooth are disunited at the base, so as to form two di- 

 verging teeth. The pit for the ligament is placed imme- 

 diately behind this angular tooth, and projects rather 

 within the shell. Lateral teeth, two on each side in one 

 valve, one on each side in the other, diverging from the 

 umbones, and placed very near the margin of the shell; 

 they are generally very thin, and in the thicker species 

 perpendicularly striated ; these lateral teeth are mostly 

 elongated, and the inner ones more prominent than the 

 outer; but in some species, M. Spengieri for example, 

 they are quite short. Muscular impressions two, lateral, 

 distant; that. of the mantle has a small sinus. Ligament, 

 consisting of two portions, (as usual) one, by far the 

 larger, internal; the other external. In some species, 

 the umbones are separated, and the ligament forms a 

 deep pit extending both within and without to the point 

 of the beaks: of this remarkable circumstance the M, 

 Spetigleri is also a strongly marked example. 



This Genus contains a great number of species, some 

 of which are handsome, and others very singular shells ; 

 upon examining a number of species, we think it might 

 be desirable to divide it into several genera, because we 

 find several distinct forms in it. Of the recent species, 

 several are common British shells; some that are com- 

 monly admitted into it are only the young shells of other 

 genera:* others are peculiar to the East or West Indies, 

 the coast of Africa, North America, &c. The fossil spe- 

 cies are not numerous, they are only found in the tertiary 

 beds, unless, indeed, some very singular fossils found 

 in the secondary strata, particularly Oolite, be truly 

 referable to this Genus; of this, however, we cannot be 

 certain, because we know not their hinges : they will be 

 found represented in Sowerby's Mineral Conchology. 



Our plate represents at 



Fig. 1. Mactra Spengieri. 



2. turgida, inside of both valves. 



3. — solida, a magnified figure to show the striated teeth. 



* M* triangularis lov Qiiidim^lG, which is a young ,«^^*ter^e. 



