OSTEOLOGY. 



originally the costals and the neurals were distinct from the ribs and neural spines. 

 Outside of the costals, extending on each side of the body from the nuchal to the 

 pygal, is a series of n bones, the peripherals, often called the marginals (i, 2, etc., 

 on the left side of fig. i). 



Fig. 2 represents the plastron of the same shell. It consists of a median bone, 

 the entoplastron (ent) and four pairs of bones, the epiplastra (epi); the hyoplastra 

 (hyo); the hypo pi astra (hypo); and the xiphiplastra (xiph). On each side, between 

 the fore and the hind leg, the hyoplastron and the hypoplastron articulate with 

 peripherals 3 to 7 inclusive to form the bridge. The free borders of the epiplastra 

 are modified so as to produce a sort of lip. The rear of the plastron is notcht. 

 The notch from which the front leg protrudes is the axillary notch, that from which 

 the hind leg emerges is the inguinal notch. That portion of the plastron in front 

 of the axillary notches is the anterior lobe, that behind the inguinal notches is the 

 posterior lobe. 



Figs. I and 2. Graptemys geographua. Xj. 



1. Carapace, c. p . I, c . p. 2, etc., the costal plates, or bones; c.s. 1, c.s. 2, etc., the costal scutes; n. I, n. 2, etc., 



the neural bones; nu. p, the nuchal plate, or bone; nu. s, the nuchal scute; py, pygal bone; spy, suprapygal 

 bone; spy. 2, second suprapygal; v. s. i, v. s. 2, etc., the vertebral scutes; 1,2,3, 12, on the right 

 side, the marginal scutes; 1,2,3 12, on the left side, the peripheral bones. 



2. Plastron, at, abdominal scute; an, anal scute; ent, entoplastron; epi, epiplastron; fern, femoral scute; g, gular 



scute; hum, humeral scute; Ayo, hyopl astral bone; hypo, hypoplastral bone; in, inguinal scute; py, pygal bone; 

 xiph, xiphiplastron; 1, 2, 3, . . . . 12, on the right side, the marginal scutes; 1, 2, 3, . . . . 11, on the 

 left side, the peripheral bones. 



Just behind the axillary notch the hyoplastron sends upward a stout process, 

 the axillary buttress, to fit into an excavation in the third peripheral and the inside 

 of the lower end of the first costal. Just in front of the inguinal notch the hypo- 

 plastron sends up a similar process, the inguinal buttress, which fits into an excavation 

 in the seventh peripheral and the inside of the lower end of the fifth costal. The 

 space on the inside of the shell between the axillary and the inguinal buttresses is 

 called the sternal chamber. 



Seen from below, each costal plate presents a broad ridge along the middle. 

 This is the true rib; and at the end of the costal this rib enters a pit in one of the 

 peripherals. In this species the peripherals over the bridges are not suturally 

 joined to the costals. At their upper ends the ribs again free themselves from the 



