553 



of the lower mammals, as e. g. in the Echidna : or it coalesces with the curtailed frontal 

 pleurapophysis zs, as in the present instance, or with the maxillary appendage n, or with 

 both these and the pleurapophysis of its own vertebra (39), when the complex ' temporal 

 bone ' of anthropotomy is the result. In most mammals the pleurapophysis (ss) retains its 

 primitive independency and rib-like form, with usually the ' head ' and ' tubercle' ; but by 

 reason of its arrested growth it has been called ' styloid ' bone or process. Sometimes it is 

 separated from the short haemapophysis (40) by a long ligamentous tract, sometimes is imme- 

 diately articulated with it, or by an intervening piece. The haemal spine (41) is usually small, 

 and always single. The rudiments of hypobranchial elements (45) are retained as diverging 

 appendages of the parieto-haemal arch in all mammals, and have received the special names 

 of ' posterior coruua,' or ' thyrohyals,' from their subservient relationship to the larynx. 



In the frontal segment the centrum (9) and neurapophyses (10) very early coalesce. Two 

 separate osseous centres mark out the, body, and each neurapophysis has its distinct centre, 

 the optic foramina (op) being first surrounded by the course of the ossification from these four 

 points. The superior development of the neurapophysial plates (10), as compared with those 

 of the parietal vertebra (), in most mammals, harmonizes with the greater development of 

 the prosencephalon ; but the chief bulk of this segment is protected by the expanded spines 

 of the frontal (11) and parietal (r) vertebrae, and the intercalated squamosal (IT). This ap- 

 pendicular piece not only fulfils some of the functions of the proper cranial neurapophyses, 

 but, likewise, the normal office of the frontal pleurapophysis (zs), in the support, viz. of the 

 distal elements of the haemal arch (49-32), which now articulate directly with *r, in place of w, 

 as in all oviparous vertebrates. The true pleurapophysis of the frontal vertebra (ss) is almost 

 restricted in the mammalian class to functions in subserviency to the organ of hearing ; is some- 

 times, as in the hog, swollen into a large bulla ossea, like the parapophyses and pleurapo- 

 physes of the cervical vertebrae of Cobitis ; is sometimes produced into a long auditory tube, 

 and sometimes reduced to the ring supporting the tympanic membrane. Yet, under all 

 these changes, since its special homology is demonstrable with 28 iu the bird (No. 1364) and 

 crocodile (No. 763), as well as with the Ideologically compound bone, M a, b, c, d, in the 

 fish (No. 148), so likewise must its general homology be equally recognised, which is so 

 plainly illustrated in the fish. The frontal heemapophysis and the corresponding half of the 

 haemal spine are connate on each side in all mammals. The arch, as in other air-breathing 

 vertebrates, has no diverging appendage. 



The nasal segment is chiefly complicated by the confluence of parts of the enormously 

 developed olfactory capsules (is), and its typical character is further masked by the com- 

 pression and mutual coalescence of the neurapophyses (M). The centrum is usually much 

 elongated, as at 13, and soon coalesces with both neurapophyses (u) and with the nasal cap- 

 sules (is). The neural spine (is) is bifid. The pleurapophysis (so) or proximal element of 

 the haemal arch of the nasal vertebra has its real character and import almost concealed by 

 the excessive development of the second element of the arch (21), which resumes in mammals 

 all those extensive collateral connections which it presented in the crocodile ; and to which 

 are sometimes added attachments to the expanded spine of the frontal vertebra, as well as to 

 that of its own segment. The pleurapophysis, however, besides its normal attachment to its 

 centrum (is), sends up a process to the orbit, in order to effect a junction with its neurapo- 



4 B 



