25 



Pterygotus would be found to correspond with those of Eurypt- 

 erus, notwithstanding the figures of Profs. Huxley and Salter, are 

 corroborated by th's specimen. The dorsal side shows the first 

 and second segments clearly separated, but the ventral side has 

 them soldered together so firmly that not the faintest trace of an 

 articulation is visible, and the specimen with eleven ventral seg- 

 ments has twelve on the dorsal surface. 



This first ( and second ) thoracic segment of the ventral side 

 contains in the longitudinal median line of its anterior half a 

 small, irregularly rhomboidal shaped plate of about one-half 

 inch in width, which has its most acute angle pointing anteri- 

 orly along the median line, where it connects by a small suture 

 with the anterior margin of the segment, which at this point 

 forms a small indentation. The posterior part of this little 

 rhomboidal plate contracts suddenly with a curve at about the 

 middle of the segment ; the two sides do not connect, however, 

 but elongate from here into a narrow strip of about 1-16 inch, and 

 gradually part again to 3-16 inch, when the posterior margin of 

 the double segment is reached, where they unite with a light 

 curve. Underlying this long, narrow portion of the appendage 

 is another part bounded by suture lines, which begin at the lateral 

 angles of the small rhomboidal plate and extend down in a 

 slightly curved line to the point where the appendage connects 

 with the posterior margin of the segment. There are no termi- 

 nal processes with the exception of a small triangular extension 

 which extends beyond the segment posteriorly. The posterior 

 part of the appendage has a smooth surface ; the rhomboidal 

 plate is scaly like the whole segment. 



The second ( or third ) ventral segment is also composed of 

 two parts which overlap each other in the middle. 



The scale-like markings on the ventral side are more abund- 

 ant and coarser than on the dorsal side. 



The anterior feet are not as perfect as could be desired, but 

 there is enough to aid us in our determination. All the feet 

 are very much thinner than the corresponding organs in Euryp- 

 terus, although otherwise similar and in having eight joints. 



