146 SHOULDER-GIRDLE AND BREAST-BONE. 



distinct from it. The meso-scapular segment has been ossified by the top of the clavicle (cl.), and 

 cannot be seen at this stage ; the " furcula " is seen to be composed of the clavicles and interclavicles 

 (see fig. 2, cl., i. cl.). The latter bone is a small wedge; the former are strongly arcuate, and 

 are very broad at the upper third. The Sternum (figs. 1 and 3) is still quite soft, but has the 

 permanent form ; it is broad above, and gradually narrows downwards or behind ; the costal 

 processes (c. p.) are subquadrate and hooked ; the coracoid grooves have thick lips, the outer lip 

 being evidently an outgrowth from the inner (see fig. 1), and the right groove is in front of the 

 left (fig. 3). The keel projects in front of the body of the Sternum (see fig. 3), and reaches 

 nearly to the end of the ento-sternum (fig. 1) ; the latter part is not differentiated from the inner 

 fork of the outer xiphisternum, but ends in a sharp mesial xiphoid process (m. x.). The ento- 

 sternum has no rostrum, the primordial division being indicated by a shallow notch (fig. 3, e. st.). 

 There are six pairs of costal condyles (figs. 1 and 3, c. c.) for the gradually lengthening sternal 

 ribs, and then come the long, narrow, external xiphoid bars, with their hooked and expanded ends 

 (e. x.). The sternal ribs (fig. 1, s. r.) are, as 1 said before, exceptional ; they are endosteally ossified 

 at first, and then acquire an ectosteal sheath. They unite by synovial joints both with the 

 Sternum and with the vertebral ribs (v. r.). Behind the first floating sternal rib there is a free 

 abdominal rib (a. r.), which, like the one in front of it, is at present in the Reptilian stage, 

 having no bony matter on its surface. 



Family" ALCIN.E." 



Example. Una troile, Linn. 



The Old-world Divers that come nearest to the Penguins are very much more typical orni- 

 thically than the latter, and belong to a very distinct Family, divisible into several Sub-families, 

 namely, the Alcina3 proper, Colymbina3, Podicipinas, &c. My dissections are of embryos of the 

 Common Guillemot, a form differing in nothing essential from Alca Torda, and A. impennis : I 

 have three stages from the egg, and also the adult. In an embryo at the end of the first third 

 of incubation, and the size of an ordinary filbert, we see what I have figured in Plate XVII, figs. 

 10 13; figs. 10 and 11 being magnified five diameters, fig. 12 eleven diameters, and fig. 13 

 fifteen diameters. The scapular shaft-bone (fig. 10. sc.) is not half the length of the whole of the 

 scimitar-shaped bar ; it does not nearly reach the base, which is still only separated from the head 

 of the coracoid (cr.) by a notch, (fig. 12, sc. cr.). The coracoid is only half the length of the 

 scapula ; but it is much broader, has a dilated head, and a very large hooked (Reptilian) epi- 

 coracoid region; both these regions are quite soft (figs. 10 and 12). The meso-scapular segment 

 (m. s. s.) is nearly as long as the coracoid (figs. 10, 12, and 13, m. s. s.), and is already receiving 

 osseous deposit, borrowed from the clavicle (cl.). It is best seen from behind (fig. 13); and 

 then we also observe that the strongly bent clavicle resembles at this stage that of a Teleostean 

 Fish or a Cyclodont Lizard (see Plate X) ; it is still separate from its fellow, and the " inter- 

 clavicle" (i. cl.) is forming at the angle. The "proximal pras-coracoid " (fig. 12, p.p. c.) is 

 definable as a wedge of very soft cartilage, between the head of the coracoid and the clavicle. 

 The Sternum (figs. 10 and 11) is well-shaped already, and even now is more typical in form than 

 what is seen in the Penguin ; its halves are well united, and the keel (fig. 11, k.) is of considerable 

 size. 



