Professor C. E. Beecher — Structure of Trilobites. 159 



Schuchert has kindly examined the original specimen, now pre- 

 served 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 (Fig. G). 



Fig. 6. — An enlarged profile of the mesosternal ridges of the preceding; from 

 a sketch furnished the writer by Schuchert. The lower represents the ventral 

 aspect. 



The correct interpretation of this specimen, as illustrated by 

 Micklehorough ^ and Walcott,^ is: That the club-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 

 venti-al membrane between the inner ends of the gnathobases 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. 



The last specimen to be noted in this connection is the individual 

 of Ptychoparia striata, already mentioned as described by Jaekel.^ 

 A reduced photographic reproduction of his figure (Fig. 7) is 

 presented here for comparison with similar structures, as described 

 in Triarthrtis, Calymene, Ceraurus, and Asaphus. From the data 

 here deduced, it would seem obvious that the specimen shows the 

 imprint of the ventral integument in the axial region, the dorsal 

 test and filling of the body-cavity having been removed. As in 

 Triarthrus, the body has suffered collapse, thus bringing the dorsal 

 and ventral walls quite near together. In the middle of each of the 

 five or six anterior ventral arches is a groove left by the solution 

 of the chitinous median apodeme, or buttress. On either side are 

 two oblique grooves limiting two subangular areas, and outside 

 of these are two other oblique grooves marking off sub-rhombic 

 areas. The grooves in each case represent the cavities left by the 

 removal of the chitinous thickenings of the membrane of the 

 trilobite. Jaekel's attempt to remove the rock filling these areas 

 naturally was ineffectual, since the latter represent the actual 

 impression of the ventral integument. Were they simply the fillings 

 of the hollow leg joints, as he claims, they should be readily detached 

 from the matrix. 



The foregoing descriptions and discussions of the character of the 

 ventral integument in trilobites would have little or no scientific 

 value, and would be about as useless as a minute analysis of the 

 nodes and tubercles on the glabella of a Phacops, were it not for 

 the fact that from them it is possible to reach some conclusions 

 1 Op. cit. - Op. cit. 3 Op. cit. 



