116 Monogra'pli of the Crustacea of the Cincinnati Grouf. 



upward, and the back downward, and that they made use of their 

 power of rolling themselves uj) into a ball as a defense against attacks 

 from above. If they possessed feet adequate for swimming purposes, 

 they must have been sufficient to make these tracks, either when the 

 trilobite was accidently thrown to the bottom of the ocean, or Avhen 

 cast on shore. Indeed, if the trilobites possessed feet either sufficient 

 for the purpose of creeping or swimming, they must have been fully 

 competent when coming into contact with the soft, blue mud, to make 

 these tracks. As it is the opinion of the learned naturalists that trilo- 

 bites had some kind of feet for the i:)urposes of locomotion, I feel justi- 

 fied in concluding that there is nothing unreasonable in supposing 

 these tracks to be the impressions of the " soft, tender feet," or hard 

 feet, as the case might have been, of an Asaphus. Such suppositions 

 are not to be regarded, however, as scientific facts, but simply as 

 sufficient to warrant the placing of the tracks provisionally with that 

 genus. If they swam in an inverted position, it is difficult to under- 

 stand what purpose some of the spines could have served, and especially 

 such as those which ornament the cephalic shield and back of the 

 Acidaspis Clncinnatiemis. I am inclined to think that if they swam, it 

 was with their back upward. 



Class Crustacea. 



The animals of this class have a hard -jointed integument, com- 

 posed of a thick, internal, spongy chorium or vascular cutis, a colored, 

 pigmental layer, and a cuticular, secreted, external layer ; these three 

 layers are at first all equally flexible and continuous ; subsequently, 

 transverse wrinkles appear, which gradually become segments, by the 

 cuticle acquiring calcareous matter, principally carbonate of lime, with 

 a little phosphate of lime and magnesia (and according to some, a 

 little " chitine," as in insects) ; these solidified bands become segments 

 by separating posteriorly from the lower layer or chorium, which re- 

 mains flexible, and permits the various [motions. ^_Each segment is 

 believed (for they can seldom be all demonstrated) to consist of six 

 pieces, two tergal pieces above (separated by \i median longitudinal 

 suture), two similar sternal pieces below, an epimerian piece on the 

 upper half of each side, and an episternal piece forming the lower half 

 of each side. In the lower Crustacea the segments are very numerous, 

 distinct, and nearly alike, but they gradually coalesce in the higher 

 types, coinciding with the condensation of the nervous centers. No 

 segment ever bears more than one pair of appendages, a fact which is 

 used to demonstrate the true number of the segments which may be 



