14 PATTEN AND REDENBAUGH. [Vol. XVI. 



esses are each split into three divisions. The neural portion 

 {n.. Figs. 5, 6, and 9) is in contact with the integument at the 

 side of the mouth, and furnishes attachment for a few muscle 

 strands going to the integument anterior to the labrum. The 

 middle {m., Figs. 5, 6, and 9) and haemal divisions {Ji., Figs. 

 5-9) both furnish attachment for muscles going to the posterior 

 side of the flexure of the oesophagus. The haemal division also 

 continues as a thin strand around the side of the oesophagus to 

 the haemal side of the carapace, anterior to the eyes. The 

 neural and middle divisions are rather stout, while the haemal 

 one is thin and joined at the base to its fellow upon the oppo- 

 site side. Thus in median longitudinal section the plastron 

 appears to terminate in a knife edge (//., Fig. 7). 



Posteriorly the plastron terminates, in the median portion, in 

 a thin membrane {z., Figs. 5-8), which runs backward and is 

 attached to the integument between the longitudinal commis- 

 sures of the ventral cord. It is continued laterally onto the 

 posterior sides of the apodemes {z., Fig. 8). 



The ventral longitudinal muscles of the abdomen are attached 

 to the posterior sides of the apodemes and to the plastron itself 

 on each side of the median line. A process {x, Figs. 5 and 8) 

 projecting neurally from each apodeme sends muscle strands to 

 the inside of the second pair of maxillae. 



Haemo-neural muscles are attached to the haemal sides of 

 the apodemes. There are also a pair of muscles inserted on 

 the posterior neural portion of the plastron (j.. Figs. 5, 7, and 

 10), passing to the integument between the longitudinal com- 

 missures, just back of the first cross-commissures. 



IV. Endosternite of Mygale. 



(PI. II, Figs. II and 12.) 



This plastron, like those heretofore described, lies between 

 the alimentary canal and the central nervous system. Its gen- 

 eral contour from the neural aspect is oval, with the longer axis 

 parallel to the long axis of the animal. 



The anterior edge is deeply indented by a bay running to the 



