C. E. Beecher — Ventral Integument of Trilobites. 169 



The earliest studies and illustrations of trilobites giving some 

 evidence of the nature of the ventral membrane are those by 

 Walcott on the genera Calymmene and Ceraurus. 'The limita- 

 tions of the ventral body walls of the animal were clearly 

 shown by a marked change in the color of the rock between 

 the white calcite filling the body cavity and the dark limestone 

 matrix. In figure 7, Plate Y, after Walcott, 10 showing a trans- 

 verse section of Calymmene 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 Calymmene are given in sections, in 

 figure 3, Plate III, of Walcott's paper, and of Ceraurus, in 

 figure 2 of the same plate. During a recent visit at the 

 Museum of Comparative Zoology, the writer examined many 

 of the sections made by Walcott during his long and successful 

 search for trilobite appendages. The structure shown in the 

 figure here given (Plate Y, fig. 7) was verified, and other sec- 

 tions were observed in which the folds were more pronounced, 

 sometimes extending as thin laminae into the body cavity, thus 

 having the character of a normal apodeme. 



The second instance to be noted, where the ventral mem- 

 brane has previously been illustrated, is a specimen of Asaphus 

 megistus Locke, first described by Mickleborough 9 from the Cin- 

 cinnati shales in Ohio. In his figure, an outline of which is 

 here reproduced (Plate Y, fig. 5), there are shown a number of 

 discontinuous longitudinal lines in the axis of the posterior 

 thoracic region. Mr. Charles Schuchert has kindly examined 

 the original specimen, now preserved in the United States 

 National Museum, and writes that the longitudinal wrinkles 

 in the axis are organic and not due to accident nor to tool 

 marks. In the best-preserved series "there are five longitudinal 

 ridges, a central one with two on each side." They appear 

 in cross section as shown in the sketch furnished by Mr. 

 Schuchert (Plate Y, fig. 6). 



The correct interpretation of this specimen, as illustrated by 

 Mickleborough 9 and Walcott, 11 is : That the chib-shaped bodies 

 lying within the axis are the gnathobases attached at the sides 

 of the axis ; the curved members extending outward from the 

 gnathobases are the endopodites ; the longitudinal ridges in 

 the ventral membrane between the inner ends of the gnatho- 

 bases are the buttresses and apodemes of the mesosternites ; 

 the slender oblique rod-like bodies shown in the right pleural 

 region in Walcott's figure are portions of the fringes of the 

 exopodites. 



