BIVALVIA. Ill 



Shell depressed but convex, subtrigonal, nearly equilateral and smooth; umbones 

 nearly mesial, unequal, and incurved ; posterior side short ; left valve compressed, its umbo 

 small ; surface with concentric, closely arranged, very fine, and irregular plications. 



The surface is very smooth, the posterior side is scarcely so much attenuated as is 

 usual in this genus, and the cardinal area is very obscurely defined. The general figure 

 approaches the Corimya tenuistriata of Agassiz, but that shell has a smaller longitudinal 

 diameter, and the umbones are not so nearly mesial. It would appear to be very rare, 

 but has occurred both in the lower or shelly, and upper portions of the Great Oolite. 



Locality. Minchinhainpton. 



MYACITES, Schlot. 



Syn. MYOPSIS, Ag. PLEDROMYA, Ag. ARCOMYA, Ag. 

 PLATYMA, Ag. HOMOMYA, Ag. 

 PANOP^EA, sp. Buvignier. PANOP^EA, sp. TfOrbigny. 



Shell elongated, umbones anterior to the middle of the valves, contiguous, depressed, 

 anterior border rounded, posterior border either rounded or truncated, both extremities 

 gaping, sometimes equally so, or the posterior aperture is the more expanded, and some- 

 times slightly reflected ; a depression more or less distinct extends from the umbones to 

 the inferior border ; ligament external and short ; test delicate, with irregular longitudinal 

 plications, and ornamented with a pellucid outer tegument, having granules disposed in 

 radiating lines. Hinge without teeth, with an elongated horizontal thickened plate, which 

 extends posteriorly to the umbones, and supports the ligament; muscular impressions 

 usually indistinct, but resembling those of Pholadomya, pallial impression with a very 

 large posterior flexure. 



Under the comprehensive term Myacites, we arrange a very extensive series of forms 

 which have been referred to Amphidesma, Lutraria, Sanguinolaria, Myopsis, Arcomya, 

 Pleuromya, Hoinomya, and Platymya; commencing in the Muschelkalk, their numbers 

 increased in the Lias, and they continued to hold a very prominent position throughout 

 the oolitic and lower portion of the Cretaceous rocks. 



From others of the Myada? which have granulated surfaces, as Gresslya, Goniomya, 

 and Anatina, they are distinguished by features which will be found under those genera. 



We regard Myacites as a form which connects Panopaja with Pholadomya, by means 

 of the more elongated forms of the latter species, and more especially by the hinge, which 

 differs from Pholadomya solely by the greater thickness and strength of the former. 



MYACITES VEZELAYI. Lajoye, Sp. Tab. XI, fig. 5, 5#. 



Syn. 



MYA VEZELAYI, D'Archiac. Mem. Soc. Geol. Fr., vol. v, tab. 24, fig. 4. 

 HOMOMYA GIBBOSA, Ag. Etud. Cret. Myes., pi. 18. 



