210 SHOULDER-GIRDLE AND BREAST-BONE. 



figs. 3, ep.). In many Rodents, when the transverse cleft either quite (Arctomys) or nearly 

 (Mus sylvaticus, Myoxus avettanarius) cuts through the cartilage, there are two epiphyses at 

 each end of each shaft-bone (Plate XXV, figs. 8 11 ; Plate XXVI, fig. 5, ep.) ; so that there are 

 five pleurostea to each stermal segment. This is well shown in Myoxus (Plate XXV, fig. 10, ep.), 

 and in Cricetus (fig. 6, ep.) ; these twin epiphyses are very apt to coalesce afterwards, as in Ilystrix 

 (Plate XX, figs. 14, 15, ep.), and in Mus minutus (Plate XXVI, fig. 8, ep.). The interclavicular 

 half of the manubrium is not pointed in the young Rabbit (Plate XXV, figs. 1, 2, p. st.), but 

 shows the remains of the primordial space between the " horns ;" in the large specimen (Plate 

 XXV, fig. 5, p. st.) this part is acute and keeled. In Dasyprocta acouchi (Plate XXIV, fig. 8, 

 two diameters) this cervical part is half ossified by the manubrial shaft (pi. o. 1), and the 

 pointed end by its own "proosteon " (pr. o.) as an epiphysis. In Helamys (Plate XIX, fig. 19), 

 the broad prse-sternum is surmounted by an interclavicular plate, which is squarish, and has an 

 ectosteal bone (pr. o.), with an epiphysis in front of it, and one intervening between it and the 

 first " pleurosteon " (pi. o. 1) : there has been some transverse segmentation here, perhaps 

 perfect, such as M. Gegenbauer describes in the Ccelogenys (see note, p. 209). 



In Cavia aperea, a month old (Plate XXIV, fig. 6, pr. o.), a squarish ectosteal plate appears 

 in front of the first sternal rib, on the base of the long interclavicular sternal plate. At this stage 

 there is no segmentation between the "proosteon " and the first " pleurosteon" (pi. o. 1), although 

 a very good fissure has appeared between the manubrium and the first meso-sternal piece (m. st. 1). 

 M. Gegenbauer (op. cit. p. 549) confounds the "proosteon" of the Cavy with the episternal series ; 

 a mistake which vitiates all his reasoning. The development of the sternal ossifications is by sym- 

 metrical "pleurostea" and "metostea" in Mus musculus (Plate XXVI, fig. 1, pi. o., m. t. o.), but 

 in Mus decumanus by azygous centres (fig. 10, pi. o.) ; in the latter the primordial fissure (p. f.) 

 closes much sooner than in the small species. I find azygous centres in Lepus (Plate XXV, figs. 

 1 and 2), and in Cavia (Plate XXIV, figs. 1, 2, and 3). The xiphoid part of the Sternum is 

 transversely oval in Lepus (Plate 1 XXV, fig. 3, x.) ; like a cheese-knife in Arvicola (Plate XXIV, 

 fig. 10, x.), and in Mus minutus (Plate XXVI, fig. 8, x.) ; but the angles are rounded off in Mus 

 sylvaticus (Plate XXVI, fig. 5, x.), and in Cavia (Plate XXIV, fig. 6, x.). In all kinds we find 

 the proximal narrow part occupied by a metosteon, and in some there is another feebler (endosteal) 

 centre in the broad cartilage (Plate XXV, fig. 3, x., and Plate XXVI, fig. 8, x.). In Arctomys 

 ludovicianus there are two remnants of the primordial fissures, as fontanelles (Plate XXIV, fig. 

 14, x. f.), and in this kind, and also in Myoxus (Plate XXV, fig. 8, x.), and Sciurus (Plate XXV, 

 fig. 14, x.), the xiphoid is spatulate, and comparatively narrow. 



Oreb" INSECTIVORA." 



Examples. Talpa, Sorex, Amphisorex, Crocidura, CItrysocJdoris, Erinaceus. 



The Marsupials, the Edentates, the Rodents, and the Insectivores, all lie in a circle round the 

 Monotremes. The cranium of the Mole and the Shrew is but little modified, in essentials, from 

 that of the Echidna, and the regions under review here will display to us the congenerous nature of 

 these Monotrematous and Insectivorous types. The scapula (Plate XXV, fig. 16; Plate XXVII, 

 figs 1, 2, 3, 10, 15, 18, 22 ; and Plate XXVIII, figs. 13) is high and narrow; has generally 



