J. D. Dana on the supposed Legs of a Trilohite. 321 



the forms exhibited on its under side, were truly what thej 

 appeared to be, locomotive organs." On account of his doubts, 

 the specimen was submitted by him durinir the past year to the 

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

 withstanding the corroboration there received, he oilVrJ'd to 

 place the specimen in my hands for examination and report. 



Besides giving the specimen an examination mvsell'. I have 

 submitted it also to Mr. A. E. Verrill, Prof, of Zoology in Yale 

 College, who is well versed in the invertebrates, and "to Mr. S. 

 I. Smith, assistant in the same department, and excellent in 

 crustaceology and entomology. We have separately and to- 

 gether considered the character of the specimen, and\vhile we 

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

 dependent judges. Our opinion has been subniitted to Mr. 

 Bilhngs, and by his request it is here published. 



The conclusion to which we have come is that the organs are 

 not legs, but the semi-calcified 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 appenda- 

 ges are articulated. 



This conclusion is sustained by the observation that in one 

 part of the venter three consecutive parallel arches are dis- 

 tinctly connected by the inteiwening outer membrane of the 

 venter, showing that the arches were plainly in the membrane, 

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

 tree 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 

 lact, 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. 



e curve in these arches, although it implies a forward ventral 

 extension on either side of the leg-bearing segments of the body, 

 tioes not appear to afford any good reason for doubting the 

 aoove conclusion. It is probable that the two prominences on 

 each arch nearest the median line of the body, which are rather 

 larJved, were points of muscular attachment for the foliaceous 

 api^^ndage it supported. 



^ >v ith the exception of these arches, the under surface of the 

 enter must have been delicately membranous, like that of the 

 Jtxlomen of a lobster or other macruran. Unless theunder s- 



j . fleshy, trdobites could not have rolled 



^ o a ball ; and the fact that this particular species of Asaphus 

 fii^ ^^ rolled may be owing to the unusual size or 



""uness, and also the position, of these slender arches. 



