SEELEY ON FOSSIL BIVALVE SHELLS. 



45 



analysis of rocks into their several zones, it is on the lower forms we 

 rely in synthetical arrangements. The Conchifera readily divide 

 rocks into Palaeozoic and Neozoic, — no form with a pallial sinus being 

 known below the Lower Secondary strata ; while, from the appear- 

 ance in them of numerous new genera, no class of animals better 

 marks the recognized systems into which fossiliferous rocks are 

 grouped. It is indispensable both to the geologist and biologist to 

 be familiar with the genera, and it is the object of this paper to 

 render the principles on which they are identified more easy and 

 exact. 



That genera are practically realities every student knows well ; 

 as such the geologist and zoologist have to do with them. And it 

 has elsewhere* been shown that between hosts of groups interme- 

 diate forms can no more be found, than can intermediate wood fill 

 the space between the forked branches of a bush. Accepting the 

 fact, the question arises, — How may genera be known ? The dis- 

 tinctive characters depend on the definition of a genus adopted. If 

 it is merely a number of nearly-related species, then the descriptive 

 characters will include those in the whole of the forms ; but if it 

 exists independently of the species, being fundamental to them, then 

 the specific characters cannot enter into the description of the genus. 

 Practically this latter view, which is adopted in the paper referred to, 

 reduces the description to a sixth of the shortest customary length, 

 while the distinction becomes clearer by pruning away much that is 

 common to other types. But even these characters are often need- 

 lessly redundant, and several common to nearly-allied groups may 

 be cancelled. 



Having in this spirit examined an extensive series of Lamelli- 

 branch shells in preparing for a course of lectures, given three years 

 ago, I found in most cases the hinge-characters among the most im- 

 portant residua so gotten ; and shortly after, while determining some 

 casts of fossils in which the hinge-teeth were the only generic charac- 

 ters seen, it occurred to me to write the teeth down in formulae like 

 those used for the teeth of mammalia. Subsequent re-examination 

 of specimens showed that in a large majority of cases these hinge- 

 formulae were almost the only characters with which the student 

 need be troubled ; and believing that it will not be found altogether 

 useless, the following list has been made of the more common and 

 remarkable forms, and arranged for facility of reference. 



The teeth sometimes vary much in the same genus, — one Cardium, 

 for instance, showing twelve teeth, and another not more than six, 

 though rudiments of the other six may generally be found ; and 

 occasionally the hinge is toothless, the teeth becoming obliterated 

 with age. The constant teeth are notated in ordinary numerals ; 



* '■ Researches on the Homologies of the Bivalve Mollusca; and therein of the Law 

 of Variation of Forms, and the Nature of Genera,' Part 1, communicated to the Cam- 

 bridge Philosophical Society, March 17, 1862, and remaining unpublished, pending 

 completion. The formula; used in this paper were then explained in a note. 



