AVICULID^. 273 



terior teeth laminar, elongated ; anterior (pedal) scar deep 

 posterior (addnctor) impression large, very exeentric. 



PTERONiTES, M'Co}^, 1844. Shell "thinner, subtriangnlar, with 

 the teeth less developed. P. angustatm, M'Coy. 



EOPTERiA, Billings. (?Euchasma, Billings.) Valves equally 

 convex, hinge with an external (?) ligament. P. typica, Billings. 

 Lower Silurian ; Newfoundland. 



ACTiNOPTERA, Hall, 1883. Differs from Pterinea in the absence 

 of striated ligamental area, and strong cardinal and lateral teeth ; 

 right valve convex. Several sp. Palaeozoic ; New York. P. 

 muricata, Hall. 



PTYOHOPTERIA, Hall, 18&3. Differs from Actinoptera in the 

 nasute anterior extremity, and large straight wing marked by a 

 strong longitudinal fold, 10 sp. Fossil, Chemung Group; 

 New York. P. Eugenia^ Hall. 



Pteronitella. Billings, 18*74, 

 'JDistr. — 3 sp. U, Silurian; Nova Scotia. P, ue/iMs^a, Billings, 

 Founded on casts showing in front of the beaks several small, 

 anterior cardinal teeth, and close beneath the hinge-line several 

 more or less elongated posterior teeth ; there is a strong anterior 

 muscular impression, and the whole structure of the hinge 

 resembles closely that of Cyrtodonta, 



Halobia, Bronn, 1830, 



Distr. — Trias ; Hallstadt, Spitzbergen, New Zealand, Cali- 

 fornia, Nevada. H. rarestriata, Mojs, 



Shell semicircular or semioval, with a straight edentulous 

 hinge-linge and almost central, scarcelj^ prominent beaks ; valves 

 rather compressed, equal, radiately ribbed, the ribs placed close 

 to the hinge-line being usually conspicuously thicker than 

 others ; wings subequal, rounded at their termination and not 

 emarginated, muscular scars indistinct. 



The original species described by Bi'onn is noted as H. salin- 

 aru7n, and is based upon an imperfect specimen, apparently 

 restored to a much oblique and inequilateral form. It has the 

 anterior wing conspicuously inflated along the hinge-margin, 

 and hollow internall3^ 



DAGNELLA, Mojsisovics, 1814. Lateral margins rounded into 

 the straight dorsal border ; cardinal margin compressed or with 

 an oblique furrow on the anterior side, indicating a slight ten- 

 denc3^ to form an obscure anterior auricle, defined by the 

 faintest possible indication of a marginal sinus. Barely sub- 

 genericall}^ distinct, D. Lommei, Wissm, (cxxx, 44), 



MoNOTis, Bronn, 1830. 

 Distr. — Trias; Hallein, Himalayas, Thibet, New Zealand, 

 United States. M. subsfriata, Miinst. (cxxx, 45). 



