1865.] AND VARIETIES OF CHITONIDH AND ACMEID. 275 
(=Lepidopleurus, Add.,) known by the sharp incisor-teeth lying 
within a projecting lip, there are three types of mantle-margin, 
which may be conveniently separated as subgenera, to aid in the 
difficult task of describing and identifying species. The typical 
forms, for which the name Ischnochiton should be retained, have the 
scales somewhat chaffy, and very finely striated. I. magdalensis 
and I. sanguineus well represent the group. But another series have 
the mantle-scales imbricate and strong, asin Chiton, Gray, (= Lo- 
phyrus, Add.,) from which they cannot be distinguished without 
dissection. For this Messrs. Adams’s name Lepidopleurus may be 
retained in a restricted sense. It is uncertain what Risso’s original 
genus was meant to include: his diagnosis applies to all Chitons 
with distinct side-areas and scaly margins. 
A third group, separated by Dr. Gray in his ‘ Guide,’ p. 182, as 
having the ‘“ mantle-scales minute, granular,”’ has been named T’ra- 
chydermon : it abounds in the Californian region. 
The specimens of LZ. adamsii were found among the duplicates 
named Chiton dispar by the Professor; one was attached to Discina 
cumingit. 
LEPIDOPLEURUS TENUISCULPTUS. 
L. ¢. “L. adamsii” simili ; olivacea, colore pallido seu intensiore 
minute variegata ; tota superficie minute granulosa; areis 
lateralibus vie definitis ; suturis plerumque albido maculatis ; 
mucrone antico, satis conspicuo, parte postica concava : intus, 
ut in “L. adamsii”’ formata. 
Variat : ¢. pallidore, ad jugum rufo-tincta. 
=Chiton dispar, C. B. Ad. no. 373, pars. 
The outside of this shell so much resembles the young of Chiton 
(Lophyrus) stokesii, that specimens may have been distributed under 
that name. Very few individuals were found. 
IsCHNOCHITON ELENENSIS (diagn. auct.). 
Extus areis centralibus clathris parallelis cire. xx. decussatis ; 
ar. lat. costis ii., validioribus, tumidis, tuberculosis: intus 
marginibus suturalibus posticis reflewis, tuberculatis, sinu ad 
jugum parvo; laminis insertionis unifissis, ad laminas sutu- 
rales anticas junctis, sinu latissimo.. Valva antica extus costis 
xil., haud validis ; intus fissuris x., dentibus acutis, subgrunda 
parva. Valva postica mucrone subpostico, depresso; parte 
postica expansa, concava, costis circ. xi. subobsoletis ; intus 
lamina insertionis circ. ix.-fissa, dentibus curtis, subgrunda 
parva, intus callosa. 
The central valves in this species are normal; but the posterior 
valve offers a transition towards Callochiton, the outside being con- 
cave posteriorly, the insertion-teeth short and the eaves callous. 
IscHNOCHITON (? var.) EXPRESSUS. 
I. ¢. “1. elenensi” simili, sed carnea; areis centr. clathris x., 
