Royal Society. 253 



in the explanation of those figures. The peculiar complexity of the 

 vertebrae of the Edentata having been, previously to the investiga- 

 tions of the author, illustrated by a comparison with those of the 

 Serpent tribe, he next enters upon the question of the precise nature 

 and extent of this analogy, and shows that, although the complex 

 joints in both are comparable to the tenon-and-mortice joints in 

 carpentry, they are produced by difFei'ent processes in the Mammal 

 and the Reptile. The zygapophyses exist in both ; to these, in the 

 Mammal, are superadded the joints developed on metapophyses and 

 anapophyses, which are below the zygapophyses ; but in the Serpents, 

 the superadded joints are on parts which the author terms the ' zygo- 

 sphene' and ' zygantrum,' and are a6ot>e the zygapophyses. Some 

 characteristic differences are next pointed out in the Ophidian ge- 

 nera Coluber, Hydrus, Naja, Crotalus, Pijtlion, and the extinct 

 genus of large serpents from British eocene strata called Palceophis, 

 The author also points out that the tenon and-mortice joints are not, 

 as was supposed, peculiar to the Ophidian reptiles, but exist in the 

 Iguana, where they are likewise due to the superaddition of zygo- 

 sphenal and zygantral articulations. 



The author finally enters upon the comparative anatomy of the 

 ' hypapophysis,' that name being applied to the process, commonly 

 exogenous, from the under "or ventral surface of the centrum, rarely 

 autogenous from the same aspect of the capsule of the notochordL 

 The modifications of the hypapophysis are exemplified in the Hare 

 and Rabbit, the Cape Jerboa, the Hydromys, the Phoca grcenlandica 

 and Leptonyx serridens, in the Hippopotamus, the Megaceros, the 

 Musk-deer, the Camel, the Giraffe, and other Ruminants. In the 

 Ornithorhyuchus the atlas is remarkable for a pair of hypapophyses, 

 like the first vertebra in the Sudis or Arapaima gigas : but the most 

 remarkable instances of the development and modification of the 

 hypapophysis are to be met with in the class of Birds. It is there 

 well-marked in the anterior cervical vertebrae, especially in the den- 

 tata, and reappears in the lower cervicals as a pair of processes, which 

 defend and sometimes encompass the carotid arteries, forming a 

 quasi-hsemal arch, as in the Pelican. The still more extraordinary 

 developments of the hypapophysis in the Aptenodytes and SphcEuis- 

 cus are specially described and illustrated by figures. The modifi- 

 cations of the same process are pointed out in some extinct Reptilia, 

 as e.g. the Crocodilus basijissus, the Mososaurus, the Iguanodon and 

 the Ichthyosaurus : in the latter the hypapophysis is exogenous in 

 the neck, as in some Lizards, and forms the so-called ' wedge-bones :' 

 the part usually called ' body of the atlas ' is serially homologous 

 with these ; the true centrum of that vertebra being the so-called 

 odontoid process. The memoir concludes with a demonstration of 

 the serial homology of the ha;mal arches of the tail, sometimes called 

 chevron-bones, and the essential distinction of the hypapophyses from 

 the haumapophyses, and at the same time from the parapophyses, with 

 .whicli the hypapophyses co-exist in the cervical and anterior thoracic 

 ■^^regions af the Crocodile. 



