MR. W. K. PARKER ON THE OSTEOLOGY OF BALANICEPS REX. 319 
Occipital Sclerotome in the Vertebrata. 
1. The centrum or basal part—the ‘ basi-occipital.’ 
2. Infero-lateral elements—the ‘ ex-occipitals.’ 
3. Supero-lateral elements—the ‘ mastoids.’ 
N.B. These elements coalesce very early with the petrosal in Mammalia generally ; 
but in the smaller Cheiroptera, e. g. Vespertilio, Plecotus, and Molossus, they 
coalesce with the ex-occipitals, and remain quite distinct from the petrosals. 
In many, if not most Birds, the mastoids are not distinct from the ex-occipitals, 
the outer margins of these bones having a ‘ periotic function.’ In Chelonia 
the mastoids are large and permanently distinct; but in the Crocodilia and 
Ophidia they coalesce very early with the ex-occipitals ; in the Lacertilia they 
are often distinct, but small; whilst in the Batrachia they are not distinct from 
the ex-occipitals. In Fishes the mastoids are often distinct, and are very large 
in the Gadide. 
4a. Supero-mesial elements—the inter-parietals. 
N.B. These are evidently often formed from a pair of lateral elements in the Mam- 
malia. The single piece in the adult is largest in the ‘ lissencephalous’’ mam- 
mals, e.g. Mus, Talpa. In Birds they do not exist. In the Abranchiate 
Reptilia the inter-parietal is single, and coalesces very early with the supero- 
sub-mesial elements. They have no separate existence in ordinary Batrachia. 
In Fishes the inter-parietal is single and generally remains distinct. 
4b. Supero-sub-mesial elements—the ‘ squama occipitalis,’ or the ‘ epiotics.’ 
N.B. These elements form the principal parts of the supra-occipital region in the 
Mammals, and may be developed from a pair of lateral centres. In Birds 
their inner margin is thin and squamous—the outer margin taking on the 
‘epiotic’ function ; they coalesce very early, or are originally single in this 
class. In the Abranchiate Reptilia they are small as compared with the 
inter-parietal, and very early coalesce with it. In Fishes they are generally 
distinct, and are filled with the upper part of the semicircular canals. 
Post-sphenoidal Sclerotome. 
1 a. The centrum or basal part—the ‘ basi-sphenoid.’ 
N.B. This developes a very long exogenous ‘ rostrum’ in typical Birds, and a short 
upper rostrum in Crocodilia and Chelonia. 
1b. Cortical inferior elements—the ‘ basi-temporals’ (nobis). 
N.B. These coalesce very early with each other and with the basi-sphenoid in 
Birds. They probably exist in the embryos of the higher Reptilia; for to us 
it is evident that in struthious Birds, and in all the cold-blooded Ovipara, 
the great ‘rostrum’ is a cortical ossification. 
2. The posterior infero-lateral elements—the ‘ petrosals.’ 
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