B. DEAN — CHIMÆRA 295 
to decide whether they were really the ancestral Chimæroids or whether 
they may not have been equally well aberrant forms of sharks. The so- 
called tritoral points which are present in these plates resemble closely 
structures in some of the early Cestraciont sharks. 
As to the anatomical evidence, we may note that the peculiar solid 
(holocephalous) character of the skull was regarded as a secondary con- 
dition even in the time of Johannes MüLcer, and that the majority of the 
much discussed anatomical features in this curious group are looked 
upon at the present time either as primitive, or as neotænial, or even as 
very highly specialized, according to the standpoint of the individual 
observer. 
The embryological evidence upon this problem, however, still remains 
to be considered, and to this I purpose to refer briefly in the present 
paper. In this connection it may be recalled by those who are interested 
in the problem of the relationships of the Vertebrates, that the middle 
and late stages of the development of the Australian Chimæroid Callo- 
rhynchus, have been lately described by ScxauinscanD, and that the 
younger stages, of the Californian Chimæra colliei, have been re- 
ferred to briefly (1903) by the present writer. At the present time it 
may be pointed out that in a number of important regards the deve- 
lopment of Chimæra indicates a remarkable degree of specialization, and 
that these conditions can only be interpreted on the ground that the 
Chimæroid fishes in the course of their evolution have departed develop- 
mentally from the simpler features of sharks. In passing, [ might refer 
to the following conditions : 
The Chimæroids are highly specialized : 
First: In the means fertilizing the eggs: in this regard witness, for 
example, the erectile frontal and ventro-lateral spines, and the peculiarly 
modified pterygopods. In the last, as in the genus ÆHarriotta or Rhinochi- 
mæra, the component elements are fused into a single cartilaginous naïl- 
shaped plate. 
Second : — In the remarkable egg caspules. These are far more highly 
specialized than in any known Elasmobranch. They are of gigantic size, 
measuring without their terminal organ for attachment relatively about 
one quarter the length of the parent fish: they are, moreover, 
oriented for the young fish at the time of hatching, having a definite 
region for its snout, trunk, tail and caudal filament, and even for its 
dorsal and ventral sides. The capsule is constructed, furthermore, so that 
a system of breathing pores will weather through the shell at the appro- 
priate time, and that an elaborate operculum will appear at the time of hat- 
ching. This capsule altogether affords an interesting case of almost perfect 
adaptation, one, by the way, whose origin it is difficult to understand on 
ihe ground of pure natural selection. For how may two morphologically 
