160 Professor C. E. Beecher — Structure of Trilohites. 



regarding the myology of trilohites, and thus add something to the 

 knowledge of their internal organization. 



In the abdomen of a normal crustacean, as is well known, there 

 is a pair of longitudinal dorsal muscles, the extensors of the abdomen. 

 They divide into bundles, which are attached on the inner surfaces 

 of the tergites of the somites. Likewise, on the ventral side, there 

 is a larger pair of longitudinal muscles, the flexors of the abdomen, 

 from which strands are a:iven off and attached to each sternal arch. 



Fig. 7. — Ftychoparia striata, Emmr. Dorsal view of the anterior portion of 

 a specimen preserved as a cast in sandstone, and enlarged about two diameters. 

 In the glabellar and anterior thoracic region, the filling of the body-cavity has 

 been removed from the axial region, thus exposing the imprint of the hypostoma 

 and ventral integument with its buttresses, or apodemal structures. Reduced 

 from the original figure published by Jaekel. Cambrian : Bohemia. 



The strands from one somite unite with the main bundles within the 

 cavity of the next anterior somite. In a diagrammatic form, this 

 disposition of the ventral musoles is represented in the accompanying 

 figure (Fig. 8). 



Now, since in Crustacea it is of very common occurrence to have 

 chitinous extensions of the integument within the body-cavity either 

 to divide or to support organs, as well as for the attachment of 

 muscles, it seems a necessary conclusion to refer the thickenings and 

 buttresses on the ventral membrane of trilohites to the same class 

 of structures, which are usually termed apodemes. With this 

 interpretation, the median longitudinal ridge on the mesosternite 



