34 PfiOF. G. B. HOWES AND ME. H. H. SWINNEETON 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 

 PI. 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 " incisurae costse " 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 praestevnal 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 1 , 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 2 ) . 



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). 



2 Baur, G. : Anat. Anz. Bd. xiv. 1897, p. 150. 



