64 THE NAUTILUS. 



vain through Dall ['79,] Fischer ['87,] Pilsbry ['92] and other 

 leading modern authorities for some other tangible distinction between 

 the two genera, but all the cited authors retain the two nearly in the 

 exact Carpenterian sense above given, though the single diagnostic dif- 

 ference results in their rather wide separation under Pilsbry's scheme. 



The trouble with this single diagnostic difference is that according 

 to my observations it does not exist in the type-species, marmorea. 



According to my observations neither ruber rior marmoreus has 

 scales, strictly speaking. In both (in ruber always, in marmorea 

 usually) the girdle is set with minute, stumpy spines. In ruber these 

 have some such shape as a thimble or short finger-cot and are thick- 

 set, giving a shagreen-like surface, while in marmorea they are pro- 

 portionately longer and thinner, commonly absolutely much smaller, 

 usually (if not always) much less thickly set, often very sparse 

 indeed and apparently sometines lacking entirely. 



In other words my observations bear out the beautiful accuracy of 

 Sars [ '78,] Tab. 8, figs. 3 a 1. and 1 a 1., on the basis of which 

 he erected the genus Boreochiton expressly to contain our two forms, 

 in ignorance of Carpenter's prior Tracliydermon. 



If Sars' figures and my observations are correct there is no more 

 ground for generically separating ruber and marmorea on the basis 

 of girdle-character [and no other has been suggested] than there 

 would be for putting No. 3 and No. 1 sandpapers in different genera. 

 If indeed the two species do not actually intergrade in this character, 

 at least the difference between the coarsest and finest (or naked) 

 marmorea is far greater than the difference between the coarsest 

 marmorea and the finest ruber. 



The upshot is that I feel confident marmorea must be removed to 

 Trachydermon and stand close beside T. ruber. What is to become 

 of Tonicella after the removal of its type-species to a prior genus I 

 leave to others. 



Admitting that ruber and marmorea stand close beside one another 

 in the same genus, how are they specifically distinguished ? I think 

 the answer must be only by what Dall calls " the sum of the char- 

 acters." Gould's supposed diagnostic differences have been dealt 

 with already. Great as is the apparent difference between the rough 

 red-and-white striped narrow girdle of an extreme ruber and the 

 smooth, leathery, green, wide girdle of an extreme marmorea they 

 almost if not entirely overlap on this point also. 



