392 GHITONID^. 



eklge of the soft parts of the several species, a separation of 

 them into groups of generic value on account of slight dif- 

 ferences in the shelly valves or ornamental surface of the 

 mantle is, to say the least, premature. On such diiFe- 

 rences, however, good sections of the genus may be founded, 

 and in such light we regard the divisions proposed by Mr. 

 Gray and others. The most important contribution to 

 a scientific knowledge of this genus is the recently pub- 

 lished Memoir on Russian Chitons by our distinguished 

 friend Dr. Middendorff, in which the species coming 

 within range of his theme are investigated with a minute- 

 ness and skill which cannot be praised too highly. This 

 author is the first to notice a ofcaracteristic of the greatest 

 value in the determination of species, but one which, being 

 taken from the soft parts, has been neglected by concho- 

 logists, viz. the number of branchial laminae and the 

 extent occupied by them. Our own observations and 

 those of Mr. Clark confirm, so far as they go, the value 

 assigned to this character by Middendorff". A still more 

 minute character — the form of the teeth, especially the 

 central one, on the lingual ribband is also of great specific 

 importance in Chiton, though difficult to observe. We 

 owe the indication of it to the admirable researches of 

 Professor Loven on the lingual teeth of Mollusca * — 

 researches which will do much towards a right under- 

 standing of the genera and families of Gasteropoda, and 

 of which we intend fully to avail ourselves in the future 

 portion of our work. 



* Ofversigt af Kongl. Vetenskaps-Academiens Forhandlingar. June, 1847. 



