ACKPHALA TKSTACKA. 



TKKJONIA, Bntg. 



So remarkable for the hinge, which is furnished with two plates en 

 ehrvron, crenulated on both faces, each of which penetrates into two 

 cavities, or rather between four plates of the opposite side, similarly 

 crenulated on their internal surface. 



The internal impressions on the shell had already warranted the 

 supposition that the animal was not provided with long tubes. Messrs. 

 Quoy and Gaymard have lately discovered living specimens of this 

 genus, and in t'.u-t. its mantle, as in the Arcae, is open and without 

 any separate orifice, even for the anus. The foot is large, its anterior 

 portion trenchant and like a hook. 



The living Trigoniae resemble the Cardiae in the form of their 

 shell, and the ribs which furrow it : its interior is composed of 

 nacre *. 



The fossil Trigoniae are different. Their shell is flattened on one 

 side, oblique, longest in a direction perpendicular to the hinge, and 

 traversed in a contrary direction by series of tubercles f. 



FAMILY II. 



MYTILACEA. 



In the second family of the testaceous Acephala, the mantle is open 

 before? but has a distinct aperture for the faeces. 



All these bivalves have a foot, used in crawling, or at least serving 

 to draw out, direct and place the byssus. They are commonly known 

 under the generic name of Muscles. 



MYTILUS, Lin. 



The true Mytili or Sea- Muscles have a closed shell, with equal, con- 

 vex and triangular valves. One of the sides of the acute angle forms 

 the hinge, and is furnished with a long, narrow ligament. The head 

 of the animal is in the acute angle ; the other side of the shell, which 

 is the longest, is the anterior one, and allows the passage of the byssus ; 

 it terminates in a rounded angle, and the third side ascends towards 

 the hinge, to which it is joined by an obtuse angle ; near this latter is 

 tin* .nus, opposite to which tho mantle forms an opening or small 

 1 artii-ular tube. The animal CALLITRICHE, Poli, has the edges of its 

 mantle provided with branched tentacula near the rounded angle, as 

 it is then- tli.it the water enters required for respiration. Before, and 

 near the acute angle is a small transverse muscle, and a large one 

 behind, near the obtuse angle. Its foot resembles a tongue. 



In the true Mytili the summit is close to the acute angle. 



Some of them are striated and others smooth. 



* The TriyonU namfe, Lam.. Ann. <lu Mas., bivii, 1. 



f Trig, scabra, Encyc. Method., pi. 237, f. 1 ; TV. nodulosa, Ib., 2 ; TV. warn, 

 ; TV. aspera, Ib. 4. See also Parkins., Or*. Rem., Ill, pi. xii. 



