1883.] Recent Literature. 393 
oped. (6) That although the cartilaginous cranium (as that of 
the shark) becomes segmented in higher animals, that segmenta- 
tion follows some law, which law may probably be found in a 
repetition in the skull of the manner in which, when the vertebral 
column becomes segmented, the lines of division pass through the 
middle of each protovertebra—thus their parachordals and trabec- 
ulz would, by median division after the union, form three portions. 
(7) That of the median bones of the base of the skull, the pre- 
sphenoid, which is a median vertebral element formed by the paired 
trabecular cartilages, that theoretically ought not to form such an 
ossification, can only be explained by the law that “the longer a 
type endures in time the more perfectly is the vertebral plan of 
that type superimposed upon the skull.” (8) That the distinction 
of “membrane bones” and cartilage bones is not one of great im- 
portance, the former arising simply from the fact that the nervous 
substance of the brain “ grows up so rapidly that the cartilage 
elements are unable to cover it.” (9) That the face originates in 
the jaws, which in elasmobranchs are separate from the skull. 
The jaws are developments of the mandibulary cartilages, so that 
embryologically the lower jaw is the most important. The facial 
bones seem, as suggested by Balfour, to form two series, the inner 
consisting of vomer, palatine and pterygoid, the outer of maxil- 
lary, inter-maxillary and jugal. *“ It is quite possible that the six 
bones of the lower jaw, which in the lower vertebrates may be 
ranged into an outer and inner series of three each, correspond to 
the inner and outer bars of the palato-maxillary region? If the 
segmentation is not carried downwards from the brain-case, It 1S 
difficult to account for it.” Yet whatever explains the segmenta- 
tion of the hyoid and branchial arches will also account for that 
of the face. Professor Seeley thinks it possible that the nasals, 
the labial cartilages of the elasmobranchs, are the basis of the 
nasals and premaxillaries. 
Why any of these cartilages, including the branchial cartilages, 
should first come into existence as they do, without any obvious 
relation to skull structure, and yet finally become the framework 
of the skull is beyond the limits of knowledge, and the only key 
(here Professor Seeley exhibits Lamarckianism equal to that of 
x X 
ag tag vanish when “ embryology becomes the servant instead of 
Grarr’s MONOGRAPH OF THE TURBELLARIANS.'—The two vol- 
par folio of text and plates (all from the author's own drawings) 
= devoted to the Rhabdoccelida, of which Professor Graff has 
Monographie der Turbellarien. 1. Rhabdoceelida, Dr. LUDWIG VON GRAFF. 
Leipzig, W. Englemann, 1882. 
