208 report — 1846. 



In urging a reconsideration of the value and significancy of these charac- 

 ters, I may repeat that in mammals the mastoid constantly presents them, 

 whilst the squamosal very rarely has the first, and not often the second cha- 

 racter. It must also be remembered that the squamosal loses its connection 

 with the frontal and progressively decreases in the mammalian class to less than 

 the dimensions of the mastoid itself, as e. g. in echidna (fig. 12), whilst in this 

 monotreme the mastoid, s, besides its connections with the parietal and exocci- 

 pital, extends forwards to articulate with the alisphenoid, 6. If ossification 

 were restricted in mammals to no. s, fig. 11, in reference to 16, which re- 

 mained cartilaginous, then no. s would have the same relation to the otocrane, 

 or in other words, would contribute the same protection to the acoustic laby- 

 rinth, which no. 8, fig. 5, performs in fishes ; the external semicircular 

 canal at least would be protected in the mastoid by both : only in mammalia 

 the mastoid would also extend over the posterior canal. The petrosal loses 

 no part of its essential character as the capsule or outer tunic of the laby- 

 rinth by becoming ossified, nor is it less recognisable in fishes within the 

 mastoid, by remaining membranous or cartilaginous, than is the sclerotic 

 capsule of the eye in its chamber or orbit; which capsule, in like manner, 

 presents all the corresponding histological modifications in one or other part 

 of the vertebrate series. The mask which has concealed the true features of 

 resemblance in the human mastoid to that of fishes, is simply the petrosal 

 ossified and cemented to it. But the squamosal presents no such relations to 

 the bony capsule of the semicircular canals in any mammal. Even the 

 connection of the squamosal with the tympanic bone is, as we have seen, far 

 less constant and intimate in mammals than the connection of the mastoid 

 with the tympanic*. 



In the anatomical description of the existing ganoid fishes which M. 

 Agassiz has unfortunately called ' Sauroidf,' the bone no. s is described as 



* Prom the remark in p. 53, t. ii. pt. ii. ' Recherches sur les Poiss. Foss.,' it would seem 

 that the circumstance of the extension of the tympanic air-cells into the mastoid, in certain 

 mammalia, had weighed with M. Agassiz iu determining its homological characters. 



t All the characters by which these highly organized fishes approximate the Reptilia are 

 found, not in the highest, hut in the lowest order of that class, viz. in the batrachia, and herein 

 more especially in the salamanders. The air-bladder of Lepidosteus resembles the lung of 

 the serpent in its singleness, and those of the salamander in the degree of its cellularity ; 

 some parts of the structure being peculiarly piscine. The bifid air-bladder of Polypterus 

 resembles the lungs of the salamandroid menopome and proteus, in the want of cellular 

 walls. The characteristic large bulbus arteriosus and its numerous rows of valves, which 

 distinguish the ganoids from most other osseous fishes, are retained in the menopome, but 

 are not present in any saurian. The anterior ball and posterior cup of the vertebrae of Le- 

 pidosteus are repeated in the salamander and pipa, but in no existing saurian. The laby- 

 rinthodont character of the teeth of Lepidosteus was developed to its maximum in the great 

 extinct reptiles {Salamandro'ides, Jager), which, by their double occipital condyle, denti- 

 gerous double vomer, and biconcave vertebras, were essentially Batrachia, not Sauria ; and 

 which combined characters now found in the lower salamandroid Batrachia, with the dental 

 ones borrowed from fishes, and but feebly manifested by the most fish-like of saurians 

 {Ichthyosaurus). All the so-called sauroid fishes retain the characteristic piscine articular 

 concavity on the basioccipital for the atlas : it is, however, very shallow in the polypterus ; 

 and is also extended transversely, with the lateral borders or angles so prominent, that, as 

 M. Agassiz well remarks, " it needs very little to change this transverse articulation with its 

 two lateral ridges into two distiuct articular condyles," /. c. p. 71. But this would convert, 

 pro tanto, the polypterus into a batrachiau, not into a saurian. So far as the character of a 

 single convex occipital condyle is valuable as a mark of affinity to the Sauria, it is present 

 in a fish of a different order from the ganoids, and with much fewer approximations in other 

 respects to the reptilian class, viz. in the Fistidaria tabaccaria. There remains, therefore, 

 only the character of the enamelled scales which the polypterus and lepidosteus present in 

 common with all the lower organized ganoids, and which to a certain extent resemble the 

 bony scutes of the crocodilia. If the deposition of calcareous matter in and upon the skin 

 were not essentially a retention of a very low type of skeleton ; if it were not presented by 



