Professor C. E. Beecher — Structure of Trilobites. 153 



axial region. The specimens hitherto described were prepared to 

 show details of the appendages, and though portions of the ventral 

 membrane were exposed in many individuals, the subject was not 

 considered of sufficient moment to warrant a distinct study, par- 

 ticularly as no characters were observed in the cuticle that had not 

 been previously seen in more or less perfection by Walcott ^ in the 

 genera Ceraurus and Calymene. A recent discovery by Jaekel,^ 

 however, necessitates the separate consideration of this structure. 

 This necessity arises from the fact that a positive addition to the 

 knowledge of the trilobite anatomy may be deduced, although, as 

 will be shown, Jaekel was apparently entirely misled in his inter- 

 pretation of the nature of his discovery. 



In the paper under discussion, Jaekel ^ states that the occasion for 

 his publication arose from the finding of a specimen of Ptychoparia 

 striata, from the Cambrian of Bohemia, in which some structures 

 were preserved in the axis of the six anterior segments of the thorax. 

 These, he asserts, are the proximal joints of the legs. 



The specimen was preserved as a cast in a rather coarse-grained 

 sandstone, and is exposed from the dorsal side. From certain 

 surface indications of lines in the cast, Jaekel was led to follow 

 these into the rock filling the axis, and succeeded in finding a central 

 groove, with two oblique grooves on each side. These he con- 

 sidered as representing the cavities left by the removal of the test 

 from the basal joints of the legs, which thus must have been 

 attached along the median line of the sternum. The supposed joints 

 of the legs were filled with rock, and his attempts to separate them 

 from the matrix resulted in failure. 



In the oral region, there were still more indefinite and obscure 

 evidences of cavities left by the removal of some ventral testaceous 

 structure. 



These meagre remains in the rachis of the thoracic and oral 

 regions have furnished data for what must be considered as the 

 most remarkable and erroneous reconstruction of the trilobite 

 appendages and anatomy that has appeared since the time of 

 Burmeister,^ in 1843. The latter, in the absence of any material, 

 confessedly based his opinions of the ventral anatomy wholly upon 

 theoretical considerations. Not only has Jaekel to a large degree 

 set aside the evidence presented by many scores of specimens of 

 Triarthriis, as described by the writer, in which each detail of 



vol. xlvi (1893). " On the Mode of Occurrence, and ttie Structure and Development 

 of Triarthrus Becki'" : Amer. Geol., vol. xiii (1894). "The Appendages of the 

 Pygidium of Triarthrus'''' : Amer. Jouru. Sci. [3], vol. xlvii (1894). "Further 

 Ohservations on the Ventral Structure of Triarthrus'^ : Amer. Geol., vol. xv (1895). 

 "The Morphology of Triarthrus" : Amer. Jouru. Sci. [4], vol. i (1896) ; Geol. 

 Mag., Dec. IV, Vol. Ill (1896). 



1 C. D. "Walcott, " The Trilobite : New and Old Evidence relating to its Organiza- 

 tion": Bull. Mus. Comp. ZooL, vol. viii, No. 10 (1881). " Aj)pendages of the 

 Trilobite": Science, vol. iii. No. 57 (1884). 



- 2 Otto Jaekel, "Beitrtige zur Beurtheilung der TrHobiten," Theil i: Zeitschr. 

 Deutsch. Geol. GeseU., Bd. liii. Heft 1 (1901). 



^ Hermann Bunneister : " Die Organisation der Trilobiten,'' etc., 1843. 



