•290 
.miall: bonks of ctenodus. 
Fig 1. — Ctenodus. Superficial Cranial Ossifications. (Occipital Region). 
The two sides are drawn from different examples, x ^. 
figure shows two slightly different arrangements met with in two 
different skulls, which possibly belong to two species. Messrs. 
Hancock and Atthey were inclined to regard such variations as 
diagnostic of particular species.'' I have found them to occur in 
every skull examined, and so far as a very limited experience is to 
be trusted at all, it renders improbable any such degree of con- 
stancy as would serve to indicate species. The general disposition 
of the membrane-bones which defended the top of the skull 
agrees with Dipterus. as Hancock and Atthey have observed ; it 
agrees pretty well with the Sturgeon also, which is here figured 
for comparison. The nomenclature (parietals, frontals, &c.) ap- 
plied to bones of the same order in the skulls of higher vertebrates, 
is not applicable, so far as can be shewn, to Fishes, and since 
these bones differ greatly both in number and arrangement within 
the class, it would perhaps be best to avoid for the present any 
such appropriation of terms to particular ossifications as might im- 
ply a well-founded theory resting upon relations of observed con- 
stancy. 
* Nat. Hist. Trans. N. & 1)., vol. iv., p. 402. 
