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cipally according to the internal characters. Mr. Henry Adams, in 

 compiling the "Genera" from H. Cuming's collection, was not 

 allowed to examine the insertion plates. He thought he saw, how- 

 ever, a correlation between the internal and external marks ; and ac- 

 cordingly redescribed Gray's genera, with lists of species, according 

 to the surface diagnosis. Gray in his " Guide" unfortunately copied 

 from H. Adams' list without examination. Lastly Chenu, as usual, 

 reproduced the mistakes of H. Adams, with fresh ones of his own. 



Having had unusual opportunities of dissecting-out the valves of 

 Chitons, I have felt compelled to rectify the previously published lists, 

 and also to propose various new genera. These I communicated to 

 Mr. Binney, while his edition of Dr. Gould's " Invertebrata " was 

 passing through the press; but he did not think well to alter the 

 position of every one of our species, as I feel compelled to do. 



1. The Chiton apiculatus does not appear in H. Adams or Gray. 

 It is a true Chcetopleura ; distinguished by the thin hairy girdle, regu- 

 lar valves with sharp teeth, and long series of gills. I have not seen 

 it alive. It ranges from southern Massachusetts to Florida. The 

 genus is for the most part tropical. 



2. The Chiton ruber is Leptochiton ruber of H. Adams ; and is prob- 

 ably Callochiton puniceus Couth, of the same author. It is the Tonicia 

 rubra of Gray's " Guide," to which he adds as synonymes in P. Z. S., 

 marmorea and fulminata ; and it also appears in Gray's "Guide "as 

 Corephiumf rubrum. It has not the characters of either of these four 

 genera, in which our two best authors have placed it. It belongs to 

 Gray's genus Ischnochiton ( = Lepidopleurus H. Ad., not Kisso) section 

 t, "mantle-scales minute, granular;" but as the gill-rows are short, 

 instead of surrounding the foot as in the typical species, it is neces- 

 sary to establish a fresh genus, Trachydermon. The insertion-plates 

 are, as in Ischnochiton and Chcetopleura, regularly slit and sharp all 

 round. Mr. Emerton first observed a great peculiarity in the animal; 

 that there is a cancellated space between the posterior gill and the 

 caudal extremity. Prof. Verrill observed that in different specimens 

 there were either one, two or three rows of holes on each side. The 

 caudal lobe is generally figured as an anal tube, but in truth it is an 

 imperforate muscle, working, the posterior part of the girdle. The 

 fseces were distinctly seen to escape, sometimes on one side, some- 

 times on the other; as it appeared to me, from a slit on each side. 



3. The Chiton albus is Leptochiton albus of H. Adams, = sagrinatus 

 Couth. I twice captured a live specimen, but each time it eluded the 

 after-search. I do not doubt that this is also a Trachydermon, but 

 cannot vouch for the peculiar characters above quoted. The genus 

 belongs, in the main, to cold and temperate seas. 



4. The British C. marginatus is also a Trachydermon and not a Lep- 



