300 CllETACEOUS PELECYPODA 



hinge Avitli one very tliiok cardinal tootli in the right valve, extending posteriori}-, 

 with one long anterior and one long posterior lateral tooth ; muscular scars one 

 on either side, each surmounted by a small accessory impression; type, Carb. 

 acuta, Sow. 



This w^ould appear to be the oldest form of UNION A CEA ; the lateral teeth 

 of the right valve (the only one as yet upon record) closely resemble those of 

 Uiiio, but the presence of a large separate cardinal tooth makes this classification 

 somewhat doubtful, for all the genera of this order seem to possess only one kind of 

 hinge-teeth, may they be called cither cardinal or lateral teeth. Another difference; 

 rests in the small impressions above the muscular scars, a character which is 

 very commonly met with in species of the Astaetid^ f Cardlnia, Astarte, &c.), 

 but not in any genus belonging to the present order. However, as these fossil shells 

 have not as yet been satisfactorily examined, the present classification may be 

 equally correct as any other I could suggest. I have no good specimens for 

 examination, and can, therefore, add nothing to the characteristic as at pi'esent 

 recorded. M°Coy says that there are about twenty species in the coal-measures 

 "associated exclusively with fresh-water and terrestrial remains." Certainly there 

 can no reasonable doubt be entertained why the fresh-waters should not have 

 been populated during the time of the carboniferous dej^osits with UyioyiojE when 

 other much higher organised animals lived in them, and again others on the 

 svirroundiug dry land. 



In external appearance Carbonicola greatly resembles Anthracosia, which I 

 may also have classed here but for reasons which will be found stated in the family 

 Saxicavid^ (see p. 8G). 



2. Frisconaia, Conrad, 1867, (Am. Journ. Conch., iii, p. 10). Shell ovate ; 

 hinge in the left valve with two cardinal teeth, the anterior compressed, angular, 

 oblique, with an anterior pit; the posterior broad, smooth, convex, triangular, 

 situated under the beak and directed posteriorly, emarginated at its end ; no lateral 

 teeth, muscular impressions situated near the cardinal line ; type, P. ventricosa. 

 Con., from carboniferous beds in Kansas, North America. 



This genus, Conrad says, bears the same relation to Carbonicola that Marga- 

 ritana bears to Vnio. The teeth certainly have in this case the form of true car- 

 dinals, for they are very close to each other below the beak, but there are no other 

 teeth in the hinge, and the anterior muscular scar is quite adjacent to the anterior 

 cardinal. There is, therefore, less objection against the classification of this genus 

 in the present family, than has been stated in the previous case. 



3. TJnlo, Philippsson, 1788. Shell generally ovately elongated, sometimes 

 sub-trigonal or rectangular, moderately tumid and of rather solid structure ; ante- 

 rior hinge-tooth double or of irregular shape, the lamella being either simple or 

 more or less corrugated and denticulated ; posterior tooth elongated, single in the 

 right, double in the left valve ; type, TJ. pictorum, Linn. 



The desirability of classifying the very numerous recent species of rnio in 

 sub-divisions or sections, and the difficulty experienced in doing this, has l)ecn 



