181 



the tip of the right antapical, and thence along its inner (left) margin to 

 the base of the right horn at the upper angle of the oblique postmargin. 

 It is thus in contact dorsally with plate 4'". Its left median margin is 

 in contact with the ventral plate [v.-p] and immediately behind the ventral 

 plate it is in contact for a short distance (Fig. 5) with the right tip 

 of the mesad projection of the dorsal antapical (2""]. 



The other plates of the postcingular series, 2"' — 4'", lie on the 

 dorsal side of the epitheca. Plate 2'" is usually a narrow plate on the left 

 margin of the hypotheca; 3'" lies to the left of the fission line. Plate 4'" 

 lies to the right of this line and extends posteriorly to the tip of the 

 right antapical horn, forming its dorsal side only. The right horn thus 

 belongs entirely to the postcingular series of plates. 



The left on the other hand is made up of a pair of posterior or 

 antapical plates (1"" and 2""), one of which (1"") is ventral and the other 

 2"") dorsal. The suture line separating these two plates lies in the mid- 

 frontal plane and is frequently marked by a primary list, but is never 

 easily followed as a suture in situ because of its position. The ventral 

 antapical plate (1"") is extended anteriorly on the ventral face of the 

 hyjjotheca till it abuts against postcingulars 1'" and 2"' and its mesad 

 margin borders the ventral plate. The dorsal antapical forms not only 

 the dorsal side of the left antapical horn but covers the dorsal face of 

 the posterior part of the hypotheca, meeting postcingular plates 2"', 3'" 

 and 4"'. A long narrow shank (Figs. 5 and 6) projects to the right, espe- 

 cially in species of the C. tripos group, forming the oblique postmargin 

 between the bases of the antapical horns and meets at its squarish tip 

 the posterior angle of postcingular 5"' of the right antapical horn. The 

 antapical plates are bounded anteriorly by the fission line to the point 

 of the suture between plates 3"' and 4'". 



The girdle is composed of four narrow trough-like plates (Figs. 1, 

 2, 1 — 4) which part most readily at the fission line in the middorsal 

 suture, and less readily in lateral sutures near those which separate pre- 

 cingulars 1" and 2" and 3" and 4". This suture and the fact that the 

 left antapical horn is composed of two plates, in so far as I can deter- 

 mine, has not been previously noted. It is one of the sutures most 

 difficult to demonstrate. 



The distinction in size between the long left antapical horn and the 

 short right one holds throughout practically all the protean species of 

 this genus. It is least apparent in the long-horned species of the C. tri- 

 ]iOs and C. macroceros groups and most pronounced in the C. fusus 

 group. This distinction in size is emphasized by the fact that the obli- 

 quity of the postmargin brings the base of the right horn anterior to 

 that of the left , and by the fact that its base comes nearer to the girdle 



