the J>v7c>5?Ve Asaphus platycephalus. 367 



to show remains of legs. Mr. Billings, while he has suspected 

 the organs to be legs so far as to publish on the subject*, has 

 done so with reserve, saying, in his paper, that " the first and 

 all-important point to be decided is, whether or not the forms 

 exhibited on its underside were truly what they appeared to 

 be, locomotive organs." On account of his doubts, the spe- 

 cimen was submitted by him during the past year to the 

 Geological Society of London ; and for the same reason, not- 

 withstanding the corroboration there received, he offered to 

 place the specimen in my hands for examination and report. 



Besides giving the specimen an examination myself, I have 

 submitted it also to Mr. A. E. Yerrill, Professor of Zoology in 

 Yale College, who is well versed in the invertebrates, and to 

 Mr. S. I. Smith, assistant in the same department, and excel- 

 lent 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 independent judges. Our opinion has been sub- 

 mitted to Mr. Billings, and by his request it is here published. 



The conclusion to which we have come is that the organs 

 are not legs, but the semicalcified arches in the membrane of 

 the ventral surface to which the foliaceous appendages or legs 

 were attached. Just such arches exist in the ventral surface 

 of the abdomen of the Macrura, and to them the abdominal 

 appendages are articulated. 



This conclusion is sustained by the observation that in one 

 part of the venter three consecutive parallel arches are di- 

 stinctly connected by the intervening outer membrane of the 

 venter, showing that the arches were plainly in the membrane 

 as only a calcified portion of it, and Avere not members moving 

 free above it. This being the fact, it seems to set at rest the 

 question as to the legs. We would add, however, that there is 

 good reason for believing the supposed legs to have been such 

 arches in their continuing of nearly uniform width almost or 

 quite to the lateral margin of the animal, and in the additional 

 fact that, although curving forward in their course toward the 

 margin, the successive arches are about equidistant or parallel, 

 a regularity of position not to be looked for in free-moving 

 legs. The curve in these arches, although it implies a forward 

 ventral extension on either side of the leg-bearing segments 

 of the body, does not appear to afford any good reason for 

 doubting the above conclusion. It is probable that the two 

 prominences on each arch nearest the median line of the body, 



* Quart. Joum. Geol. Soc. 1870, No. 104, p. 479, witli a plate giving a 

 full-sized view of the under surface of the Trilobite, a species that was 

 over 4 inches in length. 



26* 



