CHAPTER XVII 
ELASMOBRANCHII: GENERAL CHARACTERS —— PLEUROPTERYGII — 
ICHTHYOTOMI — ACANTHODEI PLAGIOSTOMI — SELACHII — 
BATOIDEI—HOLOCEPHALI 
CUAss Ih: PISCES: 
Sub-Class I, Elasmobranchii., 
In both the ancient and the modern Sharks, Dog-Fishes, and 
Rays, the exoskeleton takes the form of a more or less uniform 
investment of dermal denticles or “shagreen.” The endo- 
skeleton is wholly cartilaginous or partially calcified, and there 
are neither cartilage- nor membrane-bones. The vertebral column 
is acentrous or chordacentrous, generally with alternating basi- 
and inter-dorsal elements, and terminating in a heterocercal tail. 
The skull is usually hyostylic, very rarely amphistylic or auto- 
stylic, and the lateral halves of the primary upper Jaw (palato- 
quadrate cartilages) usually meet in a highly characteristic 
median symphysis beneath the base of the skull.  Branchial 
arches and clefts are five to seven in number, and the clefts are 
separated by complete interbranchial septa, which, as a rule, are 
continuous externally with the skin. An operculum is developed 
only in the Holocephali. A pelvic girdle is present. With rare 
exceptions the pectoral fin is uniserial. The pelvic fin is in- 
variably uniserial. The exoskeletal supports of all the fins 
consist of ceratotrichia, and, when present, the fin-spines are 
invested by enamel. Claspers are generally present in the males. 
In the surviving members of the group the nostrils retain 
their primitive ventral position. There is a conus arteriosus 
with several rows of valves. A spiracle, often furnished with a 
spiracular pseudobranch, is generally present, and, as a rule, 
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