56 DR. W. e. RIDEWOOD ON THE CRANIAL [May 3, 



Ectostecd and Endosteal Bones. — With regard to the relation 

 between ectosteal and endosteal ossifications in the skull of bony 

 fishes, there is reason for believing that the ectosteal is the more 

 primitive, and that even in the few cases in which related ecto- 

 steal and endosteal bones remain distinct, as in the postfrontal 

 and sphenotic of Amia *, the endosteal and ectosteal parts of the 

 articular bone in j4 mm, Zepidosteusf, Ara2Mim,a, Albula, JSlops, 

 Megalops, Hyodon, and Gymnarchus, the endosteal and ectosteal 

 parts of the mesethmoid of Megalops, and those of the glossohyal 

 in a great variety of forms, the endosteal ossification has been set 

 up in sympathy with the ossification taking place in the dermal 

 tissues. The process of ossification is infectious, if one may employ 

 such a term in this connection, and the increase of blood-supply, 

 and the redistribution and alteration in the character of the cells 

 and matrix in the one part is shared by the subjacent parts to a 

 greater or less degree. As other examples of such related ossifi- 

 cations, there may be mentioned the squamosal and pterotic, the 

 postfrontal and sphenotic in Teleosteans, the prefrontal and par- 

 ethmoid and the ectosteal and endosteal parts of the angular in 

 such forms as Lepidosteus and Arapaima, in which the upper part 

 of the bone articulating with the quadrate, is endosteal, while the 

 ventral surface of the bone is sculptured, and has all the appear- 

 ances of a dermal bone. Having regard, therefore, to the superior 

 antiquity of the dermal constituents of the combined bones, 

 whether these have arisen phylogenetically around sensory canals 

 or by coalescence of integumentary denticles — a matter ably dis- 

 cussed by Allis (Journ. Morph. xiv. 1898, pp. 426-431) — it is 

 preferable to adopt the names that belong to sucli superficial 

 ossifications, e. g. squamosal +, prefrontal, postfrontal §. And 



* An occasional feature only, altliovigli Traquair (' Ganoid Pislies Brit. Carb. 

 Form.,' Palaeont. Soc. 1877, p. 16) and Allis (Journ. Morph. ii. 3, 1889, p. 479) seem 

 to regard it as constant. 



t Van Wijhe (Nied. Arch. f. Zool. v. 3, 1882, pp. 268 & 281) regards thecoronoid 

 also of Lepidosteus and Amia as consisting of separable endosteal and ectosteal 

 elements, which he calls the autocoronale and the dermocoronale or suprangulare. 

 The autocoronale of Lepidosteus, however, is either a bone which is to be identified 

 with the sesamoid articular (see p. 72), or is merely a thickened part of the splenial. 

 In neither Lepidosteus osseus nor L. viridis have I been able to separate it from the 

 splenial, yet Van Wijhe {I. c. pi. 16. fig. 9, a.c.) figures it as a separate bone. The 

 autocoronale of Amia, on the other hand, the bone which is marked d by Bridge 

 (Journ. Anat. & Phys. xi. 4, 1877), is a special nodule of bone developed in Meckel's 

 cartilage in relation with the articulation between the symplectic and the mandible. 

 (The bone a of Bridge is the angular ; b and c, which 1 have never seen as two 

 separate bones, are the endosteal articular ; while the '• angular " of Bridge is the 

 ectosteal articular.) 



X In his description of the skull of G-rammicoIepis, Shufeldt (Journ. Morph. ii. 2, 

 1889, p. 280 & fig. 2) discriminates between the squamosal and the pterotic. He 

 says : " At the distal extremity of the squamosal I detect a small, flake-like piece of 

 bone, thoroughly attached, though individualised by sutural traces, which I take to 

 be the representatives of the pterotic." Since, however, he also letters the squamosal 

 and pterotic separately in his figure of such a well-known skull as that of Caranx 

 (fig. 6, p. 285), it would appear that no great importance need be attached to the 

 distinction. 



§ Cole & Johnstone (Proc. & Trans. Liverp. Biol. Soc. xvi. 1902, pp. 160-161), 

 while admitting the propriety of using the term squamosal for the dermal bone lying 



