50 THE OSTEOLOGY AND MYOLOGY 



suppression, or at least the want of distinctness, of certain parts found in tlioso crania 

 that depart less from the primitive type. Secondl)'-, as being composed of numerous 

 individual parts or pieces ; that is, of separate bones, suturally joined with contif^-uous 

 ones, but not continuous through coossification ; such parts expressing not only the divi- 

 sion of the skull into four vertebral segments, but also the distinction of the several ele- 

 ments of each segment, and of its appendages. Thirdly, as a complex whole, resultino- 

 from the coherence or confluence of all these several proximate and ultimate parts ; as 

 such forming a bony box to contain and protect the brain and organs of the special senses. 

 From the first view may be learned the ultimate, or true, or de jure morphology of the 

 skull; from the second, its proximate or apparent, or dc facto morphology; from the 

 third, its teleology, which is the aim and end of all morphology. The cranium, quoad 

 four vertebra}, represents or typifies the skull of all vertebrates; as to its individual 

 bones, the skull of all mammals ; as a whole, only itself — so far is general morphology 

 modifiable for special teleology. 



No two leaves of the forest are precisely alike ; no opossum's skull is an exact dupli- 

 cate of another's. One skull may be so described that the description will not apply to 

 any other one ; or in such a way that the terms employed will indicate the skull of any 

 vertebrate ; and each of the two methods are perfectly truthful. An account of individ- 

 ual peculiarities only would be tediously pi-ofitless ; a sumnuiry of class, ordinal, or fam- 

 ily points alone, would leave much to be desired ; a description of specific characters is 

 alone satisfactory, for these mark one entity amidst innumerable forms of animal life. 

 Specific description of a skull may be more general or special, according as more or fewer 

 relations are considered. In a general way, the relations of the opossum's skull to the 

 crania of other vertebrates may be indicated in a description of its four vertebrfc ; to 

 that of other mammals, in a notice of its different bones ; and to that of other marsupials, 

 in its consideration as a whole. It suits my purpose to take up these three lines of de- 

 scription, and in the order just indicated. 



a. Of the skull as four vertehrce {dlarthromeres^). 



V. epoicephalica.— The luemal arch rests upon the thorax ; the centrum and neural arch 

 can usually be detached entire from the antecedent vertebra, without disturbing the latter. 

 The only element that ever coossifies with the antecedent segment is the neural spine, 

 and this confluence only takes place in aged skulls. The neural spine is rarely consoli' 

 dated with the neurapophyses ; the latter, though meeting above the foramen magnum to 

 complete the neural canal without the intervention of the neural spine, remain°distinct 

 from each other, but early unite with the centrum. The purapophyses even sooner unite 

 with the neurapophyses ; coossification is completed in specimens that still show traces of 

 separation between neurapophyses and centrum. In physical characters, the occipital 



1 " Diarthromere.-A word needed to supplant ' vertebra,' segmcnt-zoonule. or somite-of single-rinjr animals (Artiru- 



in the sen.e attached to this last in this paper. 'Vertebrate' lates) is already called arthromere, of .vhich the propose! 



may be .sufficiently definitive of a back-boned animal, but word is an obvious analogue."-A^c«, York Medical Record 



•vertebra, at best a meaningless name of certain ' bone?,' is Dec.,lS70. 



ina.lociuate to express the required idea of a whole segment Although we should of course prefer to employ this term, 



of an animal constructed upon the double-ring plan, besides which we recently proposed, as above, yet it is now prao- 



benig so firmly contracted by custom that the necessary e.x- tically impossible to revise the MS. for this purpose, 

 pansion is undesirable, if not impracticable. The tvpical 



