128 CARBONIFEROUS LAMELLIBRANCHIATA. 



the latter genus. The anterior adductor muscle-scar in Parallelodon has no 

 sharp, prone edge. 



All the other characters enumerated above are those of the family Arcidse, 

 and are not of generic value. Fischer points out, in his ' Manuel de Conchylio- 

 logie,' 1885, p. 976, that Macrodon " differe de Parallelodon par ses dents 

 anterieurs oblique et non subhorizontales." The anterior teeth in Parallelodon, 

 however, are not subhorizontal, but oblique. A number of genera have been 

 described from time to time with very similar characters. Palsearca, Hall, 1858, 

 which Tate, in his Appendix to Woodward's ' Manual of the Mollusca,' 1875, p. 70, 

 says is synonymous with Megalomus, Hall, 1852; Cyrtodonta, Billings, 1858; and 

 Cypricardites, Conrad, 1841. The general characters are those of Area, but the 

 hinge has " two to eight oblique anterior teeth beneath or a little in front of the 

 umbones ; two to four remote lateral teeth, parallel with the hinge-line." These 

 lateral teeth, Fischer states, are very oblique. The distribution of Palsearca is 

 Silurian-Devonian . 



Nemodon, Conrad, from the Chalk, has the form of Macrodon ; hinge with three 

 teeth in front parallel to the hinge-line, and two long lamelliform teeth behind. 



Thus we have a series of genera, represented in several beds from the Silurian 

 to the Chalk, which have much the same external form and characters, but differ 

 slightly from each other in details of hinge character, chiefly due to a variation in 

 the obliquity and length of the teeth. There can be little doubt that, had the 

 shells from the various systems occurred in one bed, the variations in the hinge 

 could have been regarded as of specific value only. 



The difference in the hinge details of P. squamifer (Phillips), P. squamosus, 

 de Kon., and P. decussatus, M'Coy, are as great as those on which the various 

 genera enumerated above are founded. Much will remain to be done in the 

 matter of nomenclature, but no real attempt can be made in this direction until 

 the accurate description of the Lamellibranchiata of all the geological epochs has 

 been accomplished. Then a revision of all the generic names will be absolutely 

 necessary ; and I am strongly of opinion that the custom of inventing new names 

 for every form of a family which happens to occur in a different system of strata 

 will be found to be based on faulty observation, and that genera and even species 

 survived for much longer periods than is at present recognised. 



The hinge characters at once separate Parallelodon from Area, Byssoarca, and 

 Cuculldea, to which the Carboniferous shells were for the most part referred by the 

 early palaeontologists. Phillips, however, described a typical form as Modiola. 

 M'Coy referred species of this genus to no less than eight genera; but the 

 poorness of his specimens, absence of details of hinge-structure, and the retention 

 of the "genera" of previous authors probably in a very great measure account for 

 this confusion. 



