Zoological Classification. 371 



No fish, the Professor tells us, exhibits any trace of an am- 

 nion; the amphibia (frogs, etc.) have no amnion, and the urinary 

 bladder is the only representative of the allantois. Amphibia 

 and fishes have many points of resemblance; the former at some 

 period, and the latter at all periods, have organs adapted for 

 aquatic respiration, which is not the case with the vertebrates 

 above them. Amphibia which possess limbs, exhibit in their 

 structure a resemblance to those of the higher vertebrates, not 

 shown by any fish. " In all amphibia the skull articulates 

 with the spinal column by two condyles, and the basi-occipital 

 remains unossified. Furthermore the cranial peduncle, or sus- 

 pensorium, to which the lower jaw is articulated, gives attach- 

 ment to the hyoidean apparatus. These last are characters by 

 which the amphibia are sharply distinguished from the higher 

 vertebrates." 



The length to which this paper has already attained pre- 

 cludes our entering into the details by which the upper classes 

 of vertebrates are separated from each other. We still find 

 grounds of distinction in the early stages of their existence, 

 and all above the fish and amphibia possess a well- developed 

 amnion and allantois. Birds and reptiles, which at first sight 

 appear so thoroughly distinct, approach each other on exami- 

 nation; so that Professor Huxley and other comparative anato- 

 mists consider birds as an " extremely modified and aberrant 

 reptilian type." 



The mammalia stand at a considerable distance from the birds. 

 All possess an amnion, and all are allantois ; " but the latter 

 ceases to exist after a very early period, or else it is placen- 

 tiferous, and serves as a means of intercommunication between 

 the parent and the offspring. . . . The visceral arches are 

 throughout life as completely devoid of branchial appendages 

 in mammals as in birds and reptiles." The occipital condyle 

 is not single as in reptiles and birds, but double, and each 

 ramus of the lower jaw is composed of a single piece, articulated 

 directly with the squamoral bones of the skull. " The greater 

 or lesser circulations of mammals are as completely distinct as 

 in birds, and there is but a single aortic arch, the left. The 

 majority of the blood corpuscles are red, free nuclei, and these 

 are always discoidal and usually circular. The blood is hot. 

 There is a complete diaphragm and none of the bronchi end 

 in air sacs," as is the case with birds. All mammals secrete 

 milk. 



We shall recur to Professor Huxley's book on another 

 occasion. Few, if any, names could be mentioned which 

 deserve to stand higher among the present race of English 

 scientific men than his. He is amazing in industry, compre- 

 hensive in knowledge, and exact in observation. Moreover, 

 VOL. vi. — no. v. B B 



