anatinidjE. 149 



cylindrical, gaping at both extremities, greatly posteriorly; 

 beaks small, narrow, pointed, nearly contiguous, not elevated 

 and but little curved ; anterior muscular impressions oval or 

 pyriform, posterior rounded; paliial impression indistinct. 12 

 sp. Liassic and Jurassic. Europe, ilf. o6Zo?i(/a, Agass. (cviii, 69). 

 MACTROMYA, Agassiz, 1842. (Plectomya, Loriol.) Shell swollen 

 or globular, very thin, striate ; hinge without teeth — at least 

 none are visible on the internal casts ; an internal rib proceeds 

 from before the beaks obliquel}^ to the anterior margin. Creta- 

 ceous and Jurassic ; Europe. M, rugosa, Agass. (cvii, 46). 



Sedgwickia, M'Coy, 1844. 



Didr. — S. attenuata, M'Coy. Carb. ; Europe. Sil. ; N. Y. 



Shell elongated, inequilateral, anteriorly rounded and shorter, 

 posteriorly snbtruncated, moderately tumid, with incurved beaks, 

 anterior half of tlie surface ornamented with concentric sulca- 

 tions, becoming obsolete posteriorly, hinge edentulous. 



PYRENOM^us, Hall, 1852. Elongated, inequilateral, anteriorly 

 rounded, posteriorlj^ attenuated and produced, concentrically 

 striate-sulcated on the surface ; beaks tumescent, anterior 

 muscular impression deep, subanterior (posterior unknown) ; 

 hinge apparently without teeth ? 



P. cuneatus. Hall (cxx, 15). Clinton group (Middle Silurian) 

 of North America. Perhaps belongs in Nuculidfe. 



LEPTODOMUS, M'Coy, 1844. Shell oblong, somewhat trape- 

 zoid, tumid, very thin, anteriorly rounded, beaks subanterior, 

 posteriorly subtruncate and gaping, concentrically sulcated ; 

 beaks incurved, with a somewhat excavated lunule below ; hinge 

 without teeth, the posterior hinge-line more or less straight, 

 muscular and paliial impressions faint. L. fy'agilis^ M'Coy. 

 The species are all palaeozoic ; those with a median sulcus 

 extending from the beaks to the ventral edge seem to be more 

 correctly referable to Grammysia ; the typical forms greatly 

 resemble some Cj^pricardias, but are readily distinguished from 

 them by their thin shells. 



Tyleria, H. and A. Adams, 1854. 



DiHtr. — T.fragilis, H. and A. Ad. (cviii, 70). Mazatlan. 



Shell oblong, rounded in front, gaping behind, covered b};" a 

 very slight epidermis ; valves thin, nearly membranaceous ; 

 cartilage inserted in a spoon-shaped hollow ; interior of shell 

 with a layer of carbonate of lime between the spoon-shaped 

 hollow and the anterior edge ; paliial line with a profound sinus. 



Anatina, Lamarck, 1809. 

 Etyvi. — Anativus, pertaining to a duck. Lantern-shell. 

 Syn. — Laternula, Bolten, 1798. Auriscalpium, Muhlf., 1811. 

 Rhynchomya, Agass. 



