Family CHITONID^E. 



=Ischnoidea -f- Lophyroidea -f- Acanthoidea of CARPENTER. Isch- 

 nochitonidce, Lophyridce and Acanthopleuridce DALL, Blake Gastro- 

 poda, p. 415, 416. Lophyochitonidce, Chcetochitonidce (part), and 

 Chitonidce ROCHEBRUNE, Miss. Sci. du Cap Horn, vi, Zool., p.{?131- 

 143, 1889. Chiton L., FISCHER, Manual, p. 879. 



Chitons in which all of the valves possess insertion-plates slit 

 into teeth ; the insertion-plate of the tail valve similar to that of 

 the head valve. Eaves developed. 



The conclusion has been forced upon me that the three divisions 

 B, Ischnoidea, C, Lophyroidea, D, Acanthoidea of Carpenter's 

 arrangement cannot be separated by sufficiently definite characters 

 to give them the rank of families. The Ischnoidea blend with the 

 Lophyroidea through certain species of Chcetopleura, Tonicella and 

 Tonicia; and the connection with those Acanthoidea having Isch- 

 noid insertion plates is even closer. It is by no means certain that 

 the Acanthoidea constitute a natural group ; the portion having 

 pectinated insertion plates may prove more closely allied to the 

 true Chitons (Lophyroidea), whilst those with sharp, smooth inser- 

 tion plates would hold a like relation to the Ischnoidea. It is, in 

 fact, not an easy matter to decide whether such genera as Callisto- 

 chiton and Pallochiton belong to the Ischnoid or the Acanthoid 

 series, Carpenter referring them to the former, whilst my own stud- 

 ies cause me to place them in the last named group. 



In the present work I have concluded to recognize Carpenter's 

 three divisions as subfamilies, believing it better to retain a classifi- 

 cation admitted to be faulty than to institute radical changes which 

 I am at present unable to place upon an indisputable basis. The 

 regular Chitons having slit insertion plates may therefore be divided 

 into the following three 



Subfamilies : 



Subfamily ISCHNOCHITONIN.E. Insertion plates smooth or nearly 

 so, sharp, slit into teeth, which project outward on all of the valves. 



Subfamily CHITONIN^E. Insertion plates finely pectinated, blunt- 

 edged, the teeth all projecting outward. 



Subfamily ACANTHOPLEURIN^:. Insertion plates smooth and 

 acute, or pectinated and obtuse ; teeth of the posterior valve dis- 

 tinctly directed forward, not outward. 



(23) 



