1893.] NATURAL SCIENCES OF PHILADELPHIA. 287 



with the radulaof a single species would have undoubtedly revealed 

 to him. The result has been, not only has he estimated too highly 

 the constancy of minor details of the radula in single species, but he 

 has made an excessive number of so-called " generic " distinctions, 

 the names of which in many cases will simply enlarge our catalogues 

 of synonyms. 



In conclusion I may point out that the relations of the radula in 

 Lepetella to that of Lepeta, etc., offer additional reasons for thinking 

 that the Lepetidce are of the limpets those most nearly allied to nor- 

 mal or more usual types of gastropods, and also that the similarity of 

 the shell of the silurian Tryblidium to that of some recent limpets 

 ( Olana, etc.) by no means authorizes us to conclude that the soft parts 

 of Tryblidium were also similar to those of recent Patellidce. Indeed, 

 when the almost incalculable length of time intervening between our 

 days and the Silurian is considered, together with the similarity of 

 recent limpet shells which are secreted by widely different animals, it 

 is almost inconceivable that the Silurian form should have any closely 

 allied recent representative. The rhythmical manner in which the 

 adductor scars of Tryblidium are arranged in pairs, clearly indicates 

 a peculiar disposition of the organs which might, indeed, have 

 paralleled in some particulars the organization of some of the Chitons 

 of that ancient time. 



For the rest, many of the ancient limpets are represented by shells 

 which might well have belonged to Lepeta or Acmiea, yet of the 

 relations of which, as in the case of many recent limpets, we are not 

 permitted to arrive at any dogmatic opinion for want of the requisite 

 data, a deficiency which, in the case of the fossils, must remain for- 

 ever unsupplied. 



