368 NORTH AMERICAN INDEX FOSSILS. 



d n ' . Shell very strongly lamellose. 



XCII Acanthopccten. 



d // . Shell not lamellose XCI. Aviculopecten. 



7 // . Ligament in numerous grooves vertical to the hinge 



margin XCVI. Cren ipecten. 



y f/ . Ligament in a median triangular internal pit e n '. 



e". Body of shell equilateral XCVII. Pecten. 



e n '. Body of shell inequilateral CIV. Lima. 



II. Ears moderately distinct, though very small, the anterior one 



larger; no byssal notch CI. Syncycionema. 



II. Ears indistinct, merging with the rest of the shell. 



LXXIII. Pseudomonotis. 



ooo. Wing or ear one, posterior, or apparently so , mm. 



mm. Surface with radial sculpture, ribs or striae iof. 



iof. Anterior byssal opening present lo*. 



io*. Shell slightly oblique 8". 



8". Hinge with teeth ./". 



f ff . Umbonal ridge exceedingly prominent. 



LXIII. Conocardium. 



f n '. Umbonal ridge weak LVI. Byssonychia. 



8° '. Hinge without teeth LVII. Allonychia. 



io*. Shell quite oblique 9 /r . 



9 // . Wing very long ,....LV. Anomalodonta. 



9 // . Wing very slightly marked LI. Lunulicardium. 



io*. Shell vertical LXXIII. Pseudomonotis. 



iof . Anterior byssal opening absent ; also shell rather strongly 



oblique LIV. A?nbonychia. 



nun. Surface without radial sculpture nf. 



Ilf. Hinge line long II*. 



II*. Beaks narrowly pointed. Umbonal angle less than 90 . 



LXXV. Myalina. 

 11*. Beaks rather broadly pointed. Umbonal angle about 



90 10". 



10" . Wing separated from body of shell by a deep sinus. 



LXXII. Monopteria. 

 10" '. Wing not separated from body of shell by a deep 



g ff . Wing very small, narrow, and appressed. 



L. Loxopteria. 



g". Wing very broad LX. Clionychia. 



I if. Hinge line very short 12*. 



12*. Cardinal teeth present; shell long and narrow. 



LVIII. Mytilarca. 



12*. Cardinal teeth absent ; shell elongate-ovate // // . 



u" ' . Umbo very strongly incurved. ..LXXVIII. Aucella. 



11". Umbo slightly incurved LIX. Plethomytilus. 



000. Ear one, anterior nn. 



nn. Shell very elongate-triangular LXI. Aviculopinna. 



nn. Shell short-triangular or subcircular LII. Plerochania. 



II. Shell without wings 4- 



4. Hinge taxodont, i. <r., composed of many alternating teeth and sockets which 



