x RESPIRATORY ORGANS 287 
the conversion of the mandibular and hyoid arches into jaws, or 
into skeletal supports for the jaws; and posteriorly, in the 
reduction which is evident when the generality of Fishes are 
compared with such primitive Elasmobranchs as Chlamydoselachus 
and Notidanus. 
In most Fishes the concave pharyngeal margins of the branchial 
arches are fringed with a double series of either cartilaginous or 
bony tubercles or filaments, the “ gill-rakers” (Figs. 161 and 
164). The anterior row of gill-rakers on each arch usually 
interdigitate with those of the posterior row on the preceding 
arch, and in this way the two rows form a sieve-like mechanism 
to prevent any solid particles, which may enter the pharynx 
with the respiratory current of water, from passing into the gill 
clefts and clogging or otherwise injuring the branchial filaments. 
In a few Fishes the gill-rakers are enormously developed, and 
subserve a function similar to that of the baleen plates of the 
Whalebone Whales in acting as a filter for straining from the 
water the small pelagic organisms on which the Fish feeds. 
This is notably the case in the great Basking Shark (Selache 
maxima)' in which the closely-set, flattened, tapering gill-rakers 
may be so long as four or five inches, and, while somewhat 
resembling “whalebone” in appearance, have the histological 
structure of vascular dentine. The nature of the food, which in 
the stomach of one specimen examined consisted solely of an 
immense quantity of plankton, including Copepods and the larvae 
of other Crustaceans,” affords clear evidence of the great value of 
such a filtering mechanism to this Shark, and, at the same time 
offers an explanation of the striking and significant reduction in 
the size of the teeth, which, relatively to the dimensions of the 
Fish, are so small as to be almost vestigial. A similar filter has 
been observed in an extinct Selache (S. aurata)* from the 
Antwerp Crag, and also in an existing South African Shark 
(Rhinodon typieus) ;* and in the latter, as in the Basking Shark, 
is associated with a marked reduction in the importance of the 
dentition. The long slender gill-rakers of the Chondrostean 
! Turner, Journ. Anat. and Phys. xiv. 1879, p. 273. For references to other 
writers see Turner, op. cit. 
2 For this information, which was based on an examination of a specimen, 
parts of which are now in the Cambridge University Museum, I am indebted to 
Dr. Harmer. ® Van Beneden, quoted by Turner, op. cit. p. 282. 
4 Andrew Smith, also quoted by Turner, op. cit. p. 281. 
