1/4 THE CHICAGO ACADEMY OF SCIENCES. 



The jointed thoracic segments of the trilobite overlap and 

 admit of more or less motion upon one another, so that most 

 trilobites possess the power of enrollment to a greater or less- 

 degree. In some cases the body is capable of being completely 

 rolled up, the posterior margin of the pygidium coming into- 

 close apposition with the doublure of the head, thus entirely 

 concealing the ventral surface. Other forms possessed this- 

 power to but a limited degree and in some it seems to have been 

 lacking altogether. 



The ventral surface of the trilobites is usually so firmly 

 imbedded in the matrix in which the fossils are preserved, that 

 it is inaccessible for study, and the paired, jointed appendages,, 

 even when present, cannot be exposed by ordinary methods be- 

 cause of their great delicacy. Until within the last decade, 

 nearly all of our exact knowledge of the ventral surface of the- 

 trilobites and their appendages was derived from the section- 

 ing of enrolled specimens.* From a study of such material 

 it was discovered that the creatures were covered externally 

 upon the ventral side, with a thin membrane attached to the- 

 reflexed margins of the cephalon, thoracic segments and pygidium. 

 This membrane was supported by transverse processes which 

 became calcareous with age and to which the appendages were- 

 attached. More recently the structure of the ventral surface- 

 of Triarthrus becki Green, from the Utica slate near Rome, 

 New York, has been made out in great detail, and these struct- 

 ures may be taken, in the main, as typical of the trilobites as- 

 a whole. 



Each segment of the cephalon,' thorax and pygidium, ex- 

 cepting only the anal segment, carries a pair of jointed append- 

 ages, all of which, save the anterior pair, are biramous. The 

 anterior pair of appendages are the antennae, they are attached 

 at the sides of the hypostome and consist of simple, many 

 jointed flagella. Alljthe remaining paired appendages are- 

 similar in structure and exhibit but a small amount of differ- 

 entiation. Each appendage or limb consists of a basal joint 

 or protopodite from which spring two branches. The inner- 

 branch or endopodite normally consists of six joints. The 

 outer branch or exopodite has one long proximal joint with a. 

 distal multi-articulate portion; from this branch long s'etae 

 extend posteriorly and on the distal portion they are so crowded 

 as to make a continuous fringe. 



*Walcott, Bull. Mus. Comp. Zool.. Vol. 8, No. 10 (1881). 



