34 PROF. G. B. HOWES AND MR. H. H. SWINNERTON ON THE 
of sternal ribs have been hitherto described for the adult, but we find that four pairs may 
be present, as, for example, in a skeleton in the Teaching Collection of the R. College 
of Science, Dublin. Similarly, in the developmental stages, four pairs may occur, as at 
Pl. VI. fig. 6 (Stage T), or four on the left side only, as at Pl. VI. fig. 5, or on the 
right, as in specimens of Stage S in our possession. 
The earliest developmental stage of the sternum we have observed is at Q. In the 
section of this figured (Pl. VI. fig. 4) but two ribs are seen to contribute to the 
sternum, but examination of serially related sections proves that the third enters also 
into its formation; and, from what we have seen, we incline to the belief that when 
a fourth is present it also has contributed. 
In the latest stage figured, the organic continuity between ribs and sternum is still 
uninterrupted, the “ incisure coste” being of late origin. As early as Stage R the 
sulcus coracoideus (cor.') is well indicated, and anteriorly the sternal plate is prolonged 
into a couple of episternal cornua, which skirt the stem of the interclavicle and 
contribute to the formation of the sulcus, in a manner which holds good for the adult, 
but has been inadequately recognized in all previous descriptions. 
In the specimen from the Dublin Museum, this last right preesternal rib reaches 
the sternum, and its cartilaginous segment, by elongation and enlargement, has the 
characters of a normal sternal member. Its osseous segment is normal, and there are 
no accompanying changes recognizable in the ribs in front of it. We have already 
adverted to one point of interest attaching to this specimen (antea, p. 30); and if, as 
there is reason to believe from what is known of the Mesozoic reptiles, the amniote 
sternum in its ancestral condition, in which it was most certainly cartilaginous, was 
more extensive than its more specialized homologue of the living forms !, fluctuation 
anteriorly and posteriorly such as we have recorded becomes the more readily 
intelligible. 
There are no indications of any elements, median or otherwise, entering into 
the composition of the sternum, beyond those derivative of the ventral extremities 
of the ribs. 
The “« Abdominal Ribs” (Gastralia*). 
These are in number 22 to 26, and they are alternately attached to the ventral 
segments of the poststernal ribs and free, at any rate for the most part, as originally 
pointed out by Giinther. Osawa records 24, of which he remarks the first is attached 
to the first poststernal rib (98. p. 491), as are those behind corresponding to odd 
numbers, back to the 21st. The 22nd, 23rd, and 24th, with those corresponding to even 
numbers, he describes as free. We find, however, that regarding the first point there 
1 Osborn has described fossil an expanded cartilaginous sternum arising from ten pairs of ribs in the 
Mosasaur Tylosaurus dyspelor (op. cit. [antea, p. 14] p. 180). 
> Baur, G.: Anat. Anz. Bd. xiv. 1897, p. 150. 
