IIO DEAN. [Vou. IX. 
cartilage figured by Smith Woodward! for Acanthodes and 
Parexus,” 
Dentition, on the other hand, is to a noteworthy degree 
suggestive of Hybodont ; teeth often appear worn ; in lateral 
region tend to become broad and blunt; their grouping is in 
interwedging banks. In this regard Cladoselache differs from 
Chlamydoselache, whose frilling gill integuments seem the only 
striking point of comparison between modern and ancient form. 
In conclusion a modification in the arrangement of the lowest 
sharks might be proposed. Cladoselachids would naturally be 
removed from the order Ichthyotomi (including Pleuracanthids), 
Cope. And, based on the extremely primitive fin girdle and 
tail characters, which Cladoselache and Acanthodian together 
present, the writer would suggest that these forms might be 
grouped together at the base of the Elasmobranch sub-class in 
an order which might be termed Pleuropterygii, in allusion to 
the fin axis not protruding from the body wall. In such a 
group Cladoselachids and Acanthodians would take subordinal 
rank. 
This arrangement would have the merit of placing the 
Acanthodian group, which has proven difficult to adjust on 
account of its puzzling specializations, in a position where, 
from its appearance in time, one would naturally expect it, 
—near the base of the Elasmobranch stem, among more 
generalized forms. 
Divisions might thus be defined: Subclass, Elasmobranchiz ; 
ORDER JI, PLEUROPTERYGII. 
Notochord unconstricted; endoskeltal cartilages permeated 
with minute granular calcifications ; (membrane bones some- 
times occurring as in the specialized (Acanthodian) sub-order). 
Tail broadly heterocercal, lacking hypural supports in upper 
lobe ; paired fins appearing as remnants of the primitive lateral 
fold, and functioning probably as balancers, directed somewhat 
downward; the line of basalia imbedded in the body-wall, its 
tnToe Cs, Ul, spps45)35¢ 
2 The second pectoral fin spine in Diplacanthus might be explained on the 
ground of the approximation of the ventral spine next posterior. Cf. Cope, 
Am. Nat., 1890, p. 413. 
