210 NAIA. 



MYSIA. Leach. A genus composed of Tellina rotundata, 

 montagu and other similar species. 



MTTILACEA. Bl. The fourth family of LamelHbranchiata, Bl. 

 The shells are described as regular, equivalve, frequently with a 

 thick, horny epidermis. A toothless hinge and a linear ligament. 

 This family contains the genera MytUus and Pinna. 



MTTILACEA. Lam. A family belonging to the first section of 

 Conchifera Monomyaria, Lam. described as having the ligament 

 partly interior, occupying the greater part of the hinge line, 

 which is straight. The shell is rarely foliaceous. The Mytilaceae 

 cannot easily be confounded with the MaUeacese, because the 

 former are generally regular and the latter are irregular, and 

 have a thick internal coating of pearl, beyond which the external 

 coating extends. The genera may be thus distinguished : 



1. Mytiltts. Umbones terminating in a point. 'Fig. 158. 



2. Deeissika. The same, with a septiform plate. Fig. 159. 



3. MoDloi/A. Anterior margin rounded beyond the umbones. 



rig. 160. 



4. Pinna. Open at the posterior extremity. Fig. 162. 



5. LiTHODOMTJS. Cylindrical, living in holes. Eig. 161. 



M STILUS. Auct. Fam. Mytilacea. Lam. — Descr. Equivalve, 

 cuneiform, obHque, smooth, with umbones terminal, poiated, and 

 posterior side broad, rounded ; hinge linear, with a long, partly 

 internal ligament ; muscular impressions two in each valve, that 

 on the posterior side large, irregular ; that on the anterior 

 small ; paUeal impression irregular. — Obs. The Linnsean genus 

 Mytnus included the Modiolse, which differ from the Mytili in 

 the rounded anterior side; and the PinnaB, which are large 

 shells, gaping at the posterior extremity. M. achatinus, PL ix. 

 fig. 158. 



MTXOSTOMA. Troschel, 1847. Cyclostoma Peteveriana, The- 

 saurus Conchyliorum, PL xxv. fig. 100, 101. 



ISr^AEA. Gray. A genus composed of Anatina longirostrum. 

 Lam. and other similar species. PL xxiv. fig. 493, 494, 495. 



ISTAIA.. Sw. A sub-genus of Castalia, Lam. thus described : 

 " Oval cardinal teeth beneath the bosses, and deeply sulcated, 



