INVERTEBRATES. 337 



distinct auriculate, the ears being nearly obsolete. Beaks cen- 

 tral, very slightly oblique, unequal, that of the left valve often 

 elevated, gibbous, and incurved; the other very small, and 

 scarcely extending beyond the hinge line. Hinge short, nar- 

 row, edentulous ; cartilage pit under the beaks in a somewhat 

 flattened area. Byssal notch or sinus of right valve narrow, 

 deep, and separated from the hinge by a very small rudimen- 

 tary ear, which does not project beyond the margin. Adductor 

 muscular scar large and subcentral ; impressions of retractor 

 muscles, small, and placed near the beaks. Surface generally 

 ornamented with radiating, more or less vaulted or scaly costse, 

 much more distinctly marked on the left than the right valve. 



This genus has been frequently referred to Monotis of Bronn, an entirely 

 distinct Jurassic or Triassic genus, from which it was separated by one of the 

 writers in 1864. It differs completely, in form and general physiognomy, from 

 the type of Bronn's genus (Monotis Salinaria), in being very inequivalve or 

 planoconvex, not at all or very slightly oblique, and particularly in being 

 always provided with a very deep, sharply defined byssal sinus under the beak 

 of the right valve.* It is more nearly allied, as stated by the author, in pro- 

 posing the genus, to Keyserling's genus Aucella, to which Prof. McCoy referred 

 one of the typical species in 1852 ; but it differs from that Jurassic group in 

 being much less oblique, more inequivalve, and provided with radiating costse, 

 as well as in possessing a distinct cartilage pit under the beak. The latter 

 character has led Prof. King to suppose it more nearly allied to the Pectinidae; 

 but as was shown by the author, in founding the genus, its shell presents, under 

 the microscope, the prismatic structure of the Pteriidse Q=Aviculidee), and not 

 the structure characterizing the Pectinidx. 



In Europe this group is generally regarded as not dating back farther than 

 to the Permian period, but in this country we find species of it in rocks that 

 cannot be separated in any way from our Coal Measures. 



* The absence of any traces of a byssal sinus in the type of the genus Monotis, would 

 seem to indicate a difference in the habits of the species of that group, from those of 

 Eumicrotis, which, judging from their strongly defined byssal notch, were during life 

 habitually attached by a byssus. 



43 Sept. 24, 1S66. 



