May 7, 1874] 



NATURE 



this delicate tapestry has been hanging and growing, and 

 breaking and growing again, quietly in darkness, beneath 

 the grind of our carriage wheels, and yet high in air, with 

 the stream of human life flowing underneath it too. Alike 

 in the pendant stalks, on the walls, and in the mounds 

 on the floor, the prevailing colour of the crystalline in- 

 crustation is pure white. These caves in middle air have 

 been shut up from the contamination from town smoke. 

 Now and then, however, the dripping water has come 

 upon soluble iron as well as lime. Hence the mounds on 

 the floor are sometimes curiously coloured yellow, brown, 

 and red. 



" As the bridge is built of sandstone, wholly or almost 

 wholly free from lime, it is evident that the material 

 which has converted these vaults into such picturesque 

 caverns has been derived from the mortar. All rain- 

 water, as is well known, takes up a little carbonic acid 

 from the air, and of that acid there is in the air of a town 

 usually more than the normal proportion. Filtering 

 through the masonry, it dissolves the hme, carrying it 

 downward in solution, and, if made to halt and evaporate, 

 depositing it again in the form of the white crystalline 

 substance which we call spar. It would be a curious 

 question for the architect how long his masonry could 

 resist this action. Certainly, in spite of what these vaults 

 in the North Bridge reveal, the masonry of that structure 

 is to all appearance as solid and firm as ever. It is evi- 

 dently impossible, however, that the mortar, if necessary 

 at all, can be piecemeal removed without in the end 

 causing the destruction of a building." 



REPORT OF PROF. PARKER'S HUNTERIAN 

 LECTURES "ON THE STRUCTURE AND 

 DEVELOPMENT OF THE VERTEBRATE 

 SKULL"* 



III. 



IN the types already considered, the exo-skeleton con- 

 sists of small placoid scales having the structure of 

 teeth, and imbedded in the skin, but being altogether 

 irrelative to the true cartilaginous endo-skeleton. In the 

 group of fishes which form so perfect a mean between 

 these Elamobranchs and the osseous fish — the Ganoids — 

 the body is covered with close-set " ganoid " scales, 

 which consist of two layers, a deeper one of bone 

 (dermostosis), and a superficial one of enamel, covered 

 only by a thin layer of epidermis. In the head these 

 scales pass insensibly into a set of bones in close relation 

 with the chondro-cranium, and having the connections, 

 positions, &c. which characterise the roofing-bones of one 

 of the higher skulls (parietals, frontals, nasals, &c.). In 

 many cases these bones are so deeply imbedded in the 

 subcutaneous tissue as to deserve the name rather of par- 

 ostoses than otdermostoses, but are always easily removed 

 by maceration or boiling. They are evidently of an 

 entirely different nature to another series found in the 

 same skulls, but in intimate connection with the cartilage, 

 and only separable by its entire destruction. These last 

 are ossifications of the chondro-cranium, and are often 

 spoken of as " cartilage-bones ; ' the former kind have 

 only a secondary relation to the primordial skull, and are 

 known as "' membrane-bones." 



In the osseous fish both these varieties of bone appear, 

 but the investing or membrane-bones are all true paros- 

 toses developed in the deeper subcutaneous tissue, and 

 the place of the ganoid dermostoscs is taken by cycloid 

 or ctenoid scales. Still the insensible gradation between 

 scales and skull-bones is very apparent : along the side of 

 the trunk passes a series of curious tubular or grooved 

 bones containing mucous glands and known as the 

 " lateral line series ; " these, on reaching the head, branch 



• Continued from vol. iv- p. 46S. 



out SO as to produce a tree-like arrangement instead of a 

 single row, and the burrowing is now, not in a set of 

 modified scales, but in true cranial bones, some belonging 

 to the opercular apparatus, some to the series above and 

 below the eye. 



IV.— Skull of the Salmon (Salmo salar).— In the 

 Teleostean the investing bones attain a greater deve- 

 lopment than in any other group, and, in the descrip- 

 tion of the salmon's skull, will be considered before the 

 cartilage- bones which they overlie, and from which they 

 come away with great ease by maceration. 



There are, in the first place, on the upper surface of the 

 skull, three pairs of bones and a single median ossifica- 

 tion. Of these, a pair of small bones, separated from one 

 another by a considerable interval, and lying over the 

 auditory region, answer to the parietals (Fig. 7, Pa) ; a 

 much larger pair roofing over all the central portion of 

 the brain case, from the parietals behind to the nasal 

 region in front, are the Irontals (Fr) ; and a very small and 

 insignificant pair situated just above the nasal sacs the 

 nasals (Na). All these are well known from their occur- 

 rence in the higher animals ; but the bone marked S.Eth 

 (super-ethmoid), which lies between the nasals and over 

 the cartilage separating the olfactory organs, is pecuhar 

 to certain osseous fishes. 



Above the eye is a small bone, known as the 

 supra-orbital (S.Or), and below and at its sides 

 a chain of bones, deeply excavated by slime-glands, 

 the sub-orbitals (Sb.Or) ; the most anterior of these 

 (Lch) seems to answer to the lachrymal bone of 

 the higher animals. The gape of the mouth, in- 

 stead of being formed, as in the shark and ray, by the 

 naked pterygo-palatine and Meckelian cartilages, is 

 bounded entirely by membrane- bones, three in the upper 

 jaw, the pre-maxilla (Pmx), maxilla (Mx), and malar or 

 jugal (Ju), and one in the lower, ensheathing Meckel's 

 cartilage, the dentary (D). The maxilla, unlike that of 

 most typical Teleosteans is dentigerous, and takes a large 

 share in the formation of the gape. Immediately below 

 the angle of the lower jaw is situated a small bone, the 

 angular (Ang). 



Two very important parostoses occur on the under sur- 

 face of the skull, where they clamp and strengthen the 

 cartilage ; these are the vomer (Fig. 8, Vo), which bears a 

 few teeth, and the para-sphenoid (Pa.S), the enormous 

 development of which is so characteristic of the bony 

 Ichthyspsida. 



Lastly there are the bones'supporting the gill-cover, or 

 operculum proper, and branchiostegal membrane, each of 

 which has its own set of osseous strengthenings. In the 

 first set are included the opercular (Op), sub-opercular 

 (S.Op), pre-opercular(P.Op), and inter-opercular (I.Op) ; 

 in the second, the branchiostegal rays (Brs.R). The 

 operculars are also divisible into two categories ; two of 

 them — the pre- and inter-opercular — are developed in the 

 fold of skin growing irom the mandibular arch, which 

 covers the cleft (existing only in the embryo) between it 

 and the hyoid (Fig. i, p. 425, Ty.Eu), while the remainiiig 

 two belong in like manner to the operculum of the hyoid 

 arch covering the branchial slits (Fig. i, CI"). The pre- 

 opercular is interesting as being the homologue of the 

 lower part of the mammalian squamosal, and the inter- 

 opercular as representing the tympanic, the two mem- 

 brane-developed ossifications of the complex temporal 

 bone of human anatomy. The branchiostegal rays are 

 flat sabre-like bones, twelve in number, attached to the 

 hinder edge of the hyoid apparatus. In most Teleostei 

 these bones are seven slender terete rays, the four upper of 

 which are attached to the outer and the three lower to 

 the inner side of the hyoid. At the point where the 

 branchiostegal membranes of opposite sides meet below 

 the throat a median ossification is developed in the sub- 

 cutaneous tissue ; this is the so-called uro-hyal, or basi- 

 branchiostegal (B.Brs). 



