208 SHOULDER-GIRDLE AND BREAST-BONE. 



rib (v. r. 1) there is a small tongue of hyaline cartilage (figs. 4 and 5, e. cr., fourteen diameters) : 

 this is the epicoracoid ; it has already coalesced with the cartilage of the rib. In a Guine:i-pig a 

 month old (figs. 6 and?, one and one-third, and four diameters) there is a small clavicle (cl.), a 

 truncated, oval " meso-scapular" segment, and a " prae-coracoid," all near together, but far from' 

 the acromion (ac.), and from the "omosternum" (o. st.). By this time the four omosternal " beads" 

 have coalesced into a small heart-shaped plate (fig. 6, o. st.) : the epicoracoid, also (fig. C, e. cr.), has 

 become ossified by a centre of its own. In Arvicola agrestis (nearly ripe) the " meso-scapular 

 segment" above the clavicle is small ; below (Plate XXIV, fig. 9, cl. p. cr. o. st.), the prae-coracoid 

 has a pair of segments lying sternad of it; these are the omosternum (o. st.). At this time 

 there is a spindle-shaped epi-coracoid (e. cr.) in the axil of the first sternal rib; it is composed of 

 soft cartilage. In a few days after birth the lowest segment of the omosternum has become 

 converted into ligamentous tissue, and the "meso-scapular" segment has been converted into 

 bone by the upper end of the clavicle (Plate XXIV, fig. 10, cl.). The clavicle is yielding 

 bony substance to the prae-coracoid ; the omosternum (o. st.) is elongating ; and the soft 

 epicoracoids are converted into a delicate bony spindle (fig. 12, e. cr.). 



In the Domestic Mouse (nearly ripe) the beaded cartilages can all be seen (Plate XXVI, 

 fig. 1, m. sc. s., p. cr., o. st.), and also the fusiform epicoracoid, like a bud in the axil of the first 

 rib (e. cr., s. r. 1). In the adult Mouse (figs. 2 and 3, o. st.), if I mistake not, the two omo- 

 sternal moieties have coalesced, contrary to what takes place in Arvicola, but not contrary to 

 what takes place to a greater extent in Cavia. It is seen that these oraosternals are very 

 bulbous below, not pointed as in the embryo : in this part there is an endosteal bone. The 

 meso-scapular segment (fig. 4, m. sc. s.) and the prae-coracoid (fig. 2, p. cr.) resist the ossifying 

 action of the clavicle (cl.) in the adult ; but the epicoracoids (figs. 2 and 3, e. cr.) soon become 

 solid bone; they are oval, and thickened in the middle. The prae-coracoid and meso-scapular 

 segment have a very similar development in the other species of the genus Mus (see figs. 5 and G 

 M. sylvalicus, half grown ; figs. 8 and 9 M. minutus, old ; and figs. 10, 11, 12 M. decuma- 

 nus, in three' stages) ; but the omosternal segment never subdivides in M. decumanus ; it is 

 pointed below, and becomes ossified to a greater extent (fig. 12, o. st.). That ossicle has not 

 appeared in the half-grown Wood-mouse (fig. 5, o. st.), but in the old Harvest-mouse (figs. 8 and 

 9, o. st.) there are two osseous centres in each of the tear-shaped cartilages ; the lower answers to 

 that in M. musculus ; and the upper, which is unique, is curiously hooked. M. Gegenbauer noticed 

 this additional bone ('Nat. Hist. Rev.,' Oct., 1865,p. 551). In M. sylvaticus and M. minutus the epi- 

 coracoid (figs. 5, 6, 8, and 9, e. cr.) are thin, triangular, notched bones; in M. decumanus, two or 

 three days before birth, there is a small granule of soft cartilage in the costal axil (fig. 10, e. cr.) ; 

 in two or three days after birth it has become more dense, and is lanceolate (fig. 11, c. cr.) ; in 

 the two-thirds grown specimen (fig. 12) it has no separate existence, although there is a small 

 epiphysis (ep.) in the manubrial cartilage (p. st.) where it once existed. 



In the Porcupine there is a great development of these parts ; in Hystrix cristata, apparently- 

 full-grown (Plate XX, figs. 15, 16, nat. size), the meso-scapular segment is a large ovoidal flap 

 (m. sc. s.), scarcely affected by the clavicle (cl.) ; the prae-coracoid (p. cr.) has its own internal centre ; 

 and the long, stout omosternals (o. st.), cupped above and rounded below, are not ossified. 1 In 



1 I have very little doubt of the existence of a continuous band of soft, large-celled cartilage in 

 the embryo of the Porcupine ; even in the adult the intercalated clavicle has had very little influence 

 upon the cartilaginous segments. 



