190 MR. W. K. PARKER ON THE STRUCTURE AND DEVELOPMENT 
It would seem therefore that where this morphological affinity has not exerted its 
force, the study of the uncombined endoskeleton would be an easy task. This is not 
the case; for there is another simple and peculiar character about the Selachian skeleton 
and skull, namely the absence of the ossified territories found in the cartilage; thus 
Nature has removed her usual landmarks. There is a calcareous deposit; but it is 
generally distributed over the cartilage as tessellated ‘ superficial endostosis,’ a mere 
calcification of regular groups of cells, these groups having no more morphological 
meaning than the “ placoid” grains that form the shagreen in which the fish is enclothed. 
In researches so manifestly difficult and slow as this it is well to do one thing only 
at a time, and that in one part mainly. It would be out of place here to enter into a 
discussion as to the general anatomy of the Selachians: what follows will show the value 
(or the writer is much mistaken) of the study of their cranial morphology. 
The present contribution to this part of biological science is, if the whole vertebrate 
series be considered, just simply the spelling-out of a word or two that may help, with 
the addition of many such essays and spellings, towards a true reading and interpreta- 
tion of the skull, and of its relation to the rest of the skeleton. 
But the great Selachian group may well have much labour spent upon it for its own 
sake; and an attempt is here made to unite and knit into one whole the labour of the 
eradationalist on one hand, and of the embryologist on the other. 
The works whose titles now follow are for the most part gradational. The labours 
of the embryologist are quoted also; but those at hand are by one young and talented 
worker, Mr. Balfour. 
The works and papers that have been of most value to me as containing descriptions 
of adult skulls of Ichthyopsida are the following, namely :— 
Jou. Mittmr. “Vergleichende Anatomie der Myxinoiden, oder Cyclostomen, mit durchbohrtem 
Gaumen. Erster Theil. Osteologie und Myologie.” Abh. Berl. Akad. 1835, p.65 ; Wiegm. 
Arch. 1836, i. p. 245. 
T. H. Hoxtzy— 
1. Elem. Comp. Anat. 1864, pp. 162-218. 
2. “On the Representatives of the Malleus and Incus of the Mammalia in the other Verte- 
brates.” Proc. Zool. Soc. May 27, 1869. 
3. “On the Structure of the Skull and of the Heart of Menobranchus lateralis.” Proc. Zool. 
Soe. March 17, 1874. 
4, “Contributions to Morphology.—Ichthyopsida. No.1. On Ceratodus forsteri, with Obser- 
vations on the Classification of Fishes.”” Proc. Zool. Soc. Jan. 4, 1876. 
or 
. “On the Nature of the Cranio-facial Apparatus of Petromyzon.” Journ. Anat. & Phys. 
vol, x. pp. 412-429, pl. 17, 18. 
Ramsay H. Traquair. “ On the Cranial Osteology of Polypterus.” Journ, Anat. & Phys. vol. v. 
pp. 167-183, pl. 6. 
