MUSCULAR IMPRESSIONS are the marks or areas formed on the interior surface of shells by the muscular fibres which attach the animals to them. Lamarck has divided his Conchifera into two kinds: 1st. Monomyaria, those which have but one adductor muscle, and consequently have but one impression in each valve, as the common Oyster, fig. 180; 2nd. The Dimyaria, those which have two, and consequently have two impressions in each valve. There are other smaller impressions in some shells besides the principal. The palleal impression is a mark or scar passing near the margin of the shell. See Introduction. MYA. Auct. _Fam._ Myaria, Lam. Pyloridea, Bl.--_Descr._ Transverse, oval, thick, gaping at both extremities, rounded anteriorly, acuminated posteriorly; hinge with one large, dilate, compressed tooth in one valve, and a suture in the other, containing the cartilage; muscular impressions two, distant, large, irregular; palleal impression with a large sinus.--_Obs._ Mya may be known by the large, prominent, broad tooth in one valve. In Anatina there is one in each valve, and, in Lyonsia, accessory pieces. Lutraria has cardinal teeth and a ligamentary pit. Few species of Mya are known. They belong to the Northern Hemisphere. M. truncata, fig. 71. MYCETOPODA or MYCETOPUS. D'Orb. _Fam._ Nayades, Lam.--_Descr._ Shell elongated, soleniform, inequivalve, inequilateral, gaping anteriorly; muscular impressions very complex.--_Obs._ These shells are said to terebrate like Pholas. Fig. 151. M. solenoides. MYARIA. Lam. A family belonging to Lamarck's order Conchifera Dimyaria. Containing the following genera: 1. ANATINA. Ligament in a spoon-shaped prominence on the hinge of each valve, shell thin. Fig. 69. 2. MYA. Spoon-shaped prominence in one valve; shell thick. Fig. 71. 3. ANATINELLA. A spoon-shaped process in both valves. Fig. 70. 4. LYONSIA. An internal bony appendage on the hinge. Fig. 491, 492. 5. MYOCHAMA. Flat valve attached, a bony appendage on the hinge. Fig. 73. 6. CLEIDOTHÆRUS. Deep valve attached, a bony appendage. Fig. 75, 76. 7. CUMINGIA. Ligamentary pit in both valves, spoon-shaped. Fig. 87.