H. Woochcard — On the Structure of Trilohites. 293 



showing that the arches were plainly in the membrane as only a 

 calcified portion of it," is surely au error arising from the supposi- 

 tion that the matrix represented a part of the organism. As no 

 membrane whatever is preserved, it can hardly be cited to prove the 

 so-called legs to be semicalcified sternal arches imbedded in it. 



Prof. Dana suggests that the supposed joints were points of 

 muscular attachment for the foliaceous appendage it supported. The 

 legs of the Crustacea are usually bifid (giving rise to an endopodite 

 and an exopodite, see Plate VIII., Fig. 2a), but they take their rise 

 from a common base, not in a double series on each side- 

 Assuming the appendages to have been soft foliaceous gill-feet, 

 then the presence of strong sternal arches would have been needless ; 

 on the other hand, if we accept Burmeister's decision, that such 

 hardened sternal arches must accompany horny or calcareous feet, 

 then it follows as a matter of course that — accepting Prof. Dana's 

 interpretation — Asaphus must have had such hard appendages, which 

 the very presence of semicalcified arches in the ventral integument 

 proves to have existed. 



In the mean time, however, seeing that these appendages really 

 look more like feet (as at first suggested by Mr. Billings) than like 

 semicalcified sternal arches (as now suggested by Prof. Dana), would 

 it not be more philosophical to consider them to be feet, and seek to 

 discover whether a semicalcified sternal arch underlies them? 



Eventually it may be proved that a very thin and almost mem- 

 branous sternum may have sufficed to give support even to chitinous 

 or calcareous limbs, provided they were assisted by apodemata 

 developed from the under side of the tergal arches, as in Limulus. 



To this suggestion the fact of the strongly trilobed form of the 

 tergal plates in many genera of Trilobites, and the development of 

 spines and tubercles upon the segments, which doubtless served as 

 points for muscular attachment within, lends considerable probability. 



Prof. Dana observes, "unless the under surface were in the main 

 fleshy, Trilobites could not have rolled into a ball." To this I en- 

 tirely assent, but I do not see that the presence of slender ambulatory 

 legs would have prevented this operation from taking place ; but it 

 should be borne in mind that many genera of Trilobites never rolled 

 up, so that amongst them, as in the modern Isopoda, the habit was 

 the exception, not the rule. 



As to the regularity of the position of the limbs, this is not ex- 

 traordinary among Fossil Crustacea, when preserved in a suitable 

 matrix, those, for example, which occur in the Lithographic stone of 

 Solenhofen, frequently presenting a most wonderful and perfect state 

 both of preservation and arrangement of limbs, antennse, etc. 



Looking at the Crustacean class as a whole, it seems to be a 

 constant rule that the test and the appendages have a direct 

 relation to each other (save in Parasitic forms — in the Cirripedia, 

 and among the Anomoura). Thus, if the general covering of the 

 body be hard and calcareous, the limbs are covered in a similar 

 envelope. If the crust be chitinous, the limbs are also chitinous or 

 soft and membranous. Seeing, then, that the Trilobita — many of 



