OSTEOLOGY. g 
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 11 bones, the peripherals, often-called the marginals (1, 2, etc., 
_ on the left side of fig. 1). 
Fig. 2 represents the plastron of the same shell. It consists of a median bone, 
the entoplastron (ent) and four pairs of bones, the eprplastra (epi); the hyoplastra 
(hyd); the hypoplastra (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. “Fhe-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 Jobe, that behind the inguinal notches is the 
posterior lobe. 
Figs. 1 and 2. Graptemys geographica. X}. 
1. Carapace. c. p.1,c. p.2, etc., the costal plates, or bones; c. s. 1, c. s. 2, etc., the costal scutes; m. I, m. 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. 1, v. s. 2, etc., the vertebral scutes; 1,2,3, . - . - 12,0n the right 
side, the marginal scutes; I,2,3,.-..- - 12, on the left side, the peripheral bones. 
2. Plastron. ab, abdominal scute; an, anal scute; ent, entoplastron; ep/, epiplastron; fem, femoral scute; g, gular 
scute; hum, humeral scute; hyo, hyoplastral bone; hypo, hypoplastral bone; im, 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 
