16 



\ N a TO M V A N I) I'll rSIO LOGY OF 



necting the plastron and carapace are much more strongly involute than we have 

 observed in any other species. When the shell is separated, they appear like four 

 projecting keels directed inwards each other, the front ones looking inwards and 

 backwards, and the posterior ones looking inwards and slightly forwards. The con- 

 cavities thus formed before and behind, external to the keels, lodge projecting por- 

 tions of the lungs. The anterior and posterior keels projecting into the space usually 

 occupied by the air sacs, deeply indent them, and cause them to present alobulated 

 appearance, which they retain when removed from the shell. Besides these four 

 large indentations, there are smaller ones, in the edge of the lungs, one in front and 

 two or more between the keels. To the inner side and behind the posterior keel 

 lies the large posterior lobe occupying chiefly the flank space immediately above 

 and in front of the inspiratory muscle. The reticulations of its interior structure 

 are much larger and more coarsely marked than those of other parts of the lung. 



The anterior belly of the expiratory muscle arises from the vertebral margins of 

 the third and fourth intercostal spaces, and from the carapace in a line diverging at 

 an angle of 30° from the spine for the space of two inches, crossing the fourth rib. 

 From this origin the fibres cover the front of the lung, the anterior and interior ones 

 being distributed as in P. rugosa, and owing to the intrusion of the anterior keel upon 

 the lung, the external fibres are displaced, so to speak, with the portions of lung to 

 which they belong, which portions lie immediately back of the ridge or keel so often 

 referred to. The largest band of those lateral fibres finds its way between the two 

 lobules of the lung which lie first and second in order of succession behind the ridge. 

 The posterior belly arises from the pelvic fascia, from the eighth and seventh verte- 

 brae, and from a curved line whose convexity looks forwards, and which terminates 

 in front of the posterior projecting keel about two and a half inches above the pos- 

 terior angle of junction of the carapace and plastron. This line is rendered more 

 sharply convex at its external third by the projection inwards of the keel alluded to. 

 The muscular fibres curve around the posterior part of the lung. The inner ones for 

 half the width of the muscle are about two and a quarter inches long; and from this 

 point they increase gradually in length to the external edge, where they are longest, 

 extending forward in a tongue-like band about five and a half inches. In the single 

 specimen examined we found on the left side a few additional fibres reaching forwa n Is 

 and inwards at least two inches beyond the main body of the muscle. The inspira- 

 tory muscle arises as in P. rugosa. It has a linear central tendon, bifid at its pos- 

 terior extremity, the shorter terminating arm being external. Into the tendon and 

 its branches the muscular fibres are inserted as in other species. 



In Graptemys geograjphica, Ag., the inspiratory muscle is, in its general features, 

 the same as described in other species, and differs only in not having even a central 

 raphe, the convergent fibres interlacing at the middle of the muscle in an imperfect 

 network which serves to replace the tendon usually found in this situation. The an- 

 terior belly of the expiratory muscle arises from the vertebral margin of the third 

 and fourth intercostal spaces, and continuously from the costal margin of the third 

 space nearly its entire circumference, and from the surface of the fourth and fifth ribs. 

 The lines of origin diverge at an angle of 30° from the anterior margin of the third 

 intercostal space, and in this specimen, the inner line bordering the spine measures 



