Professor C. E. Beecher— Structure of Trilobites. 157 



The ridges are clearly produced by a thickening of the ventral 

 integument, and can be seen when viewed from the dorsal side 

 of a specimen in which the dorsal test and filling of the body-cavity 

 have been removed. They are thus ■partly or wholly of the nature^ of 

 apodemes, or plates of chitin, which pass inward from the mesosternitea 

 and divide as well as support internal organs, and they are not, 

 therefore, in any sense the proximal joints of legs. Besides serving 

 in this manner they were doubtless efficient in giving the necessary 

 firmness to the ventral arches for the attachment of mnscles. 





^'' ^ Nf *■'"' 



Fitj. 3. — TriartJmts Becki, Green. The ventral side of the middle thoracic region 



of the specimen illustrated on Plate XI ; shomng the ends of the pleurotergites 



on the outside, with the joints of the endopodites within the pleural regions, 



and the gnathobases extending obliquely inward in the axis. The sternal arches 



with their longitudinal ridges and the interarticular membranes are represented. 



Enlarged four and one-fourth diameters. The extensions of the limbs beyond 



the carapace are omitted. 



Were these observations confined wholly to the specimens of 



Triarthrus, there might still be some chance of error, although it 



is believed that the evidence presented by this genus alone is quite 



sufficient. Additional data, however, will now be given, regarding 



other genera and families of trilobites, described independently by 



other authors, and with no intention of representing the detailed 



characters of the ventral arches. In the search for trilobite appendages 



by various investigators, the ventral membrane has naturally been 



of secondary consideration, and in the case of Jaekel's work was 



of no consideration whatever. 



The earliest studies and illustrations of trilobites giving some 

 evidence of the nature of the ventral membrane are those by 

 Walcott on the genera Calymene and Ceraurus. The limitations 

 of the ventral body-walls of the animal were clearly shown by 

 a marked change in the colour of the rock between the white 

 calcite filling the body-cavity and the dark limestone matrix. In 

 Fig. 4, after Walcott,^ showing a transverse section of Calymene in 

 the thoracic region, it is seen that the membrane in the axis, or 

 the mesosternite, is marked by four distinct lobes representing 

 cross sections of longitudinal folds, and also that the legs are clearly 

 attached at the sides. These folds can in no way be construed as 

 proximal joints of legs. The gnathobases in Calymene are given 



1 Op. cit. 



