90 DEAN. [Vou. IX. 
the first arch. The number of the branchial arches cannot be 
definitely stated. In one example five gill-slits are to be deter- 
mined, and a sixth and seventh are certainly suggested. The 
arches are directed sharply caudad and their distal (ventral) ends 
could not have been widely separated. The anterior arches 
were the stoutest and largest; the foremost pair of slits appears 
to have been connected ventrally by a loose isthmus-flap as in 
Chlamydoselache. In a number of specimens the branchial 
region is marked by a series of parallel lines, usually trans- 
verse, suggestive of fibrous filament bearers, or more probably 
of supports for loose branchial-flaps. This peculiar character 
of striation is certainly found in no other portion of the fossils 
than in the branchial region, z.e., anterior to pectorals and 
posterior to the region of mandibular articulation. The 
laminae appear to have been long, and possibly favor the 
deduction of Garman!7 as to the protrusion of gills in the 
ancient Cladodont. 
THE PAIRED FINS, AND THEIR SIGNIFICANCE. 
The mode of origin of the paired fins, as most recently 
stated by Wiedersheim,!° is in no little way explained by the 
fin-type of the Carboniferous and Permian genera, Xenacanthus 
and Pleuracanthus. In view of the studies upon these forms, 
the fin-structure of Cladoselache becomes of especial interest 
as clearly a more primitive form. Wiedersheim, commenting 
on the studies of Déderlein and Fritsch on Pleuracanthus and 
Xenacanthus, notes in summary that the structures of the pec- 
toral fin of these forms are equivalent to those of the ventral 
of the most primitive of recent selachians (the structure of 
ventral being of course more primitive than that of the cor- 
responding pectoral), and shows that the ventral in these fossil 
genera illustrates the simplest of known conditions in a paired 
fin. In Cladoseclache present material shows that the pectoral 
indicates a more primitive condition than even the ventral 
of Xenacanthus, and offers in a most remarkable way actual 
proof of the proposition that the ventral in Xenacanthid, 
arranged after the uniserial type, demonstrates that the 
pectoral too must originally have been of this form. 
