300 ANATOMY OF VERTEBRATES. 



In Sirenia, the htemapophyses, fig. 188, h, 3, are connected by 

 pleurapophyses, ib. j^U 2, completing the h?emal arch with the dia- 

 pophyses d, i ; and the Batrachian condition of pelvis (Vol. I. p. 48, 

 p. 163, fig. 101, d) is resumed. In the rest of the class two or 

 more segments following the lumbar series become, like those 

 in the head, the seat of modifications by anchylosis of the cen- 

 trums, flattening and broadening with an expanse of the neural 

 canal: and with these modifications is associated great develope- 

 ment of the hajmal arches of two of those segments, fig. 187, 

 4 and 5, which, therefore, have got special names, as ^ ischium,' 

 ib. 4, and ^ pubis,' ib. 5. These are, however, connected with 

 their respective segments by a concomitant expanse of the single 

 pleurapophysial element, fig. 188, j)l, which, so modified, has the 

 name of 'ilium,' and in some Lyencephala (Sloths, Megatherioids, 

 Armadillos) resembles that bone in Birds, by the number of sacral 

 seorments with which it articulates or coalesces. 



The caudal segments in Mammals are characterised by the 

 abrupt cessation of the pleurapophysial developement forming the 

 ilium, by the retention of the riblet, or beginning of the pleura- 

 pophysis, anchylosed, as a diapophysis, fig. 188, d, and by the 

 approximation of the ha3mapophyses, ib. h, 3, to the under surface 

 of the centrum, c, as at A, 4, the divergent bases articulating there- 

 with, and the apices converging to unite with, or develope, a haemal 

 spine, ib. 5. The wider pelvic ha3mal canal encompassed terminal 

 parts of the generative and intestinal canals ; the narrower caudal 

 one has only to defend the main blood-vessels of the tail. The 

 terminal caudal vertebrae are progressively reduced in size and 

 complexity, and vary greatly in number : anchylosis is an excep- 

 tion (^Dasypus, e.g.) in this region. 



§ 176. General Characters of the Skull. — Pursuing the survey 

 of the Mammalian modifications of the Vertebrate archetype as 

 they appear in the segments of the skeleton forming the skull, 

 with the light of the stage of developement manifested in an 

 immature Mammal when a certain growth has proceeded from the 

 several points of ossification established in the primordial mem- 

 branous and cartilaginous basis, we find that the neural arch of 

 the occipital vertebra, fig. 189, Ni, i, 2, and 3, agrees with that of 

 the Bird and Crocodile in the connation of the diapophysis, 4, 

 with the neurapophysis, 2 ; but the process, called ' paroccipital,' 

 now descends from the lower part of the arch, and, in many 

 Mammals, is of great length. An articular condyle is developed 

 from each neurapophysis, 2, which articulates with the concave 

 anterior zygapophysis of the atlas, and is the homotype of the 



