A TURTLE 131 



B. Turtles with poorly ossified shell, covered with a leathery skin and not 

 with plates. 

 Amyda Spimfera, the soft-shell turtle. Carapace round, olive-brown 

 in color and spotted or blotched; plastron while; aquatic. 



Two specimens will be needed for a complete dissection,— one 

 for the study of the outer form, the viscera including the heart and 

 the main blood vessels, the sympathetic and spinal nerves, and the 

 skeleton ; and one for the brain and cranial nerves and the smaller 

 blood vessels. The sex of the turtle may usually be determined 

 by the shape of the plastron, which in the male is flat or concave, 

 and in the female slightly convex. To kill the animal, place it 

 under a bell jar together with a wad of cotton soaked with chloro- 

 form or ether; several hours may elapse before it is dead. During 

 the progress of the dissection the animals can be kept in a 2 per 

 cent solution of formalin, or in cold storage. 



Observe the form and color of the animal ; note if the color 

 markings are bilaterally symmetrical. Turtles are sharply dis- 

 tinguished from other vertebrates by the possession of a hard case 

 or shell within which the animal can withdraw the head, tail, and 

 legs more or less completely. The shell is composed of flattened 

 plates of bone overlaid by thin plates of horn (tortoise shell), and 

 is made up of two distinct portions, the dorsal convex carapace, 

 and the ventral flattened plastron. In some turtles these two parts 

 of the shell are not in contact with each other ; in others they are 

 joined at the sides of the body. 



The horny plates forming the outer surface of the carapace fall 

 into several longitudinal rows. In most turtles the middle row con- 

 sists of five neural and one nuchal plate ; the latter is the foremost 

 one in the row, and is wanting in some turtles. Lateral to this row 

 on each side are four costal and twelve marginal plates. The pos- 

 terior pair of marginals often become fused and form the unpaired 

 pygal plate. The plastron is usually covered by two rows of horny 

 plates, with an additional pair on the bridge joining the carapace 

 and plastron on each side. 



In certain species of turtles concentric lines of growth may be 

 seen in the horny plates of the shell, each ring representing a 

 year's growth. 



