204 STUDIES IN EVOLUTION 



The features described in the present paper have been ob- 

 tained by further work on the material secured for the Yale 

 Museum by Professor Marsh. No detailed review of the 

 ventral anatomy of Triarthrus will be given at this time, 

 only such additional characters as have been observed since 

 the publication of the last paper on this trilobite. The 

 precise structure and relations of the organs here described 

 must also be left subject to slight modifications required by 

 researches which are still in progress. The writer has care- 

 fully prepared the specimens and made the drawings from 

 camera-lucida outlines. The appendages, however, are often 

 so faintly preserved or so obscure that in order to represent 

 them in a pen-drawing it is necessary to emphasize their 

 limits and their prominence, and this may sometimes lead 

 to errors of interpretation. It seems almost unnecessary to 

 state that errors are not due to any preconceived notions of 

 trilobite anatomy, since from the beginning of these investi- 

 gations it has not been possible to predict with safety the 

 exact form and details of any of the appendages. Even 

 their presence has been more or less doubtful until revealed 

 by a fortunate discovery. 



The paired appendages of the cephalon will be taken up in 

 their order, beginning with the most anterior; next the newly 

 observed characters of thoracic legs ; then the organs in the 

 median line, — the hypostoma, mouth, metastoma, and anal 

 opening. 



Close observation of the si^ecimens thus far prepared for 

 the purpose of showing the under side of the head fails to 

 detect more than five pairs of appendages attached to the 

 cephalon, apparently corresponding to the five typical limbs 

 of the crustacean head. Considerable difficulty is experi- 

 enced in determining from the ventral side of the specimens 

 the posterior limit of the cephalon. The ventral membrane, 

 which alone is usually visible, does not show marked evi- 

 dence of segmentation, and the observer is guided chiefly by 

 the margin of the cephalon, the extremities of the pleura, 

 and obscure transverse lines on the axial membrane. In a 



