C. E. Beecher — Ventral Integument of Trilobites. 171 



termed apodemes. With this interpretation, the median longi- 

 tudinal ridge on the mesosternite of a trilobite would indicate 

 the line of division between the two main ventral bundles. 

 The first pair of oblique ridges on each side would delimit the 

 main bundles and side strands, and show that these strands 

 joined the main bundles obliquely within the cavity of the 

 next anterior somite, as in ordinary Crustacea. This accounts 

 for the anterior truncation of the triangular area between the 

 median and lateral ridges in the trilobite. 



The nature of the outside pair of oblique ridges is not so 

 plain. They may serve to divide the side ventral strands of 

 the flexors from the bundle of muscles running from the prox- 

 imal joints of the legs to the dorsal test, or they may simply 

 mark the outside of the lateral strands. 



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Figure 1.— Diagram of the axial portions of three segments ; showing the 

 ventral abdominal muscles, the flexors, represented as two heavy longitudi- 

 nal lines, together with the lateral strands attached to the sternal plate in 

 each somite and continuing obliquely forward to their union with the main 

 bundle in the cavity of the next anterior somite. 



The apodemes in general seem more strongly developed 

 anteriorly in the thorax. Possibly, this condition may be 

 explained on the basis that the ventral pair of the great flexor 

 muscles received new strands at each segment from behind 

 forward, so that near the cephalon they became large bundles 

 for which progressively larger apodemes were formed. 



It may be remarked, in conclusion, that a similar though 

 apparently much simpler apodemal arrangement would be 

 developed if the musculature of the trilobites agreed with that 

 of the theoretical crustacean ancestor, or that existing in some 

 Isopods, Amphipods, etc., in which there are no large longi- 

 tudinal bundles, but motion between the somites is effected by 

 strands running from one segment to the next anterior. If 

 viewed in this manner, there would necessarily be two median 

 and two lateral strands. The previous explanation seems to 

 be more in accordance with the structures actually seen in the 

 trilobites, which in general possessed the power of enrollment 

 to a high degree, and would be expected to have had a well- 

 developed and efficient system of ventral muscles. 



