H. Woodtvard — On the Structure of Trilohites. 291 



the hypostome was not only preserved in situ (as is usually the case 

 in the Trilobita), but also the remains (more or less well preserved) 

 of eight pairs of legs corresponding with the eight segments of the 

 thorax, to the under side of which they had been attached. The 

 appendages take their rise close to the central axis of each segment, 

 and all curve forwards, and are thus most probably ambulatory 

 rather than natatory feet. They appear to have had four or five 

 articulations in each leg. Three small ovate tubercles on the pygi- 

 dium may, perhaps, indicate the processes by which the respiratory 

 feet were attached.^ The figure of Mr. Billings's Asaphus has been 

 reproduced in the accompanying Plate VIII., Fig. 1., from the 

 Quart. Journ. Geol. Soc, vol. xxAd., pi. xxxi., fig. 1. 



In offering an explanation as to the large number of Trilobites 

 which have been examined unsuccessfully, Mr. Billings suggested 

 that only the most perfectly preserved specimens are likely to 

 exhibit the organs on the under side of the body ; just, in fact, those 

 specimens which a collector would be least willing to cut into slices 

 in search of appendages. Hence, probably, the want of success 

 which has attended previous researches. 



Whilst the Canadian Trilobite was in England I had the advantage 

 of studying the original specimen, and fully concurred in the views 

 pu.t forward by Mr. Billings ; indeed I felt unable to offer any more 

 satisfactory explanation than that proposed by him ; whilst the dis- 

 covery of similar appendages in another Asaphus from the Trenton 

 Limestone, in the British Museum, tended greatly to confirm them. 



In a note upon this specimen communicated to the Geological 

 Society, and published at the same time, I ventured to suggest that 

 the evidence put forward by Mr. Billings tended to place the 

 Trilobita near to, if not in, the Isopoda Normalia : and that we 

 might fairly expect to find that the Trilobita represented a more 

 generalized type of structure than the modern Isopods.^ 



Since the interesting specimen, described by Mr. Billings, has 

 been returned to America, it has undergone a further examination 

 by Prof. Dana and others, an account of which appears simulta- 

 neously in the Annals and Magazine of Natural History for May, 

 p. 366, and in Silliman's American Journal for May, p. 320, a copy 

 of which is subjoined. 



" Besides," writes Prof. Dana, " giving the specimen an examina- 

 tion myself, I have submitted it also to Mr. A. E. Verrill, Professor 

 of Zoology in Yale College, who is well versed in the invertebrates, 

 and to Mr. S. J. Smith, assistant in the same department, and ex- 

 cellent in crustaceology and entomology. We have separately and 

 together considered the character of the specimen ; and while we 

 have reached the same conclusion, we are to be regarded as inde- 



1 See Geological Magazine, 1870, Vol. VII., p. 291. 



2 Dr. Bigsby's specimen in the British Museum not only shows the remains of three 

 pairs of appendajfes united along the raedinn line by a longitudinal ridge, but also 

 what I have good reason to believe to be the remains of one of the palpi which has 

 left its impression upon the side of the hypostome (see Quart. Journ. Geol. Soc. xxvi. 

 1870, p. 487, woodcut, fig. 1). 



