No. 2.] GEOLOGY AND MINERALOGY. 229 



case of appendages in the Asaj^hus assumed by Mr. Billings to 

 possess ambulatory legs, is, that the said appendages were merely 

 the semicalcified arches in the integument of the sternum to which 

 the true appendages were attached. 



A comparison, which these gentlemen have themselves sugges- 

 ted, between the abdomen of a Macrourau Decapod and the 

 Trilobite in question, is the best refutation of their own argument. 



The sternal arches in question are firmly united to each tergal , 

 piece at the margin, not along the median ventral line. If, then, 

 the supposed legs of the Trilobite correspond to these semicalcified 

 arches in the Macrouran Decapod, they might be expected to lie 

 irregularly along the median line, but to unite with the tergal 

 pieces at the lateral border of each somite. In the fossil we find 

 just the contrary is the case; for the organs in question occupy a 

 definite position on either side of a median line along the ventral 

 surface, but diverge widely from their corresponding tergal pieces 

 at each lateral border, being directed forward and outward in a 

 very similar position to that in which we should expect legs Qtot 

 sternal arches) to lie beneath the body rings of a fossil crustacean* 

 The presence, however, of semicalcified sternal arches presupposes 

 the possession of stronger organs than mere foliaceous gill-feet; 

 whilst the broad shield-shaped caudal plate suggests most strongly 

 the position of the branchias. In the case of the Trenton Asa- 

 2)has I shall be satisfied if it appears, from the arguments I have 

 put forward, that they are most prohahly legs — feeling assured 

 that more evidence ought to be demanded, before deciding- on the 

 systematic position of so large a group as the Trilobita from only 

 two specimens. 'i' 



With regard to the embryology and development of the modern 

 King-Crab {Limnlus poli/phemus) ^ we must await the conclusions 

 of Dr. Anton Dohrn before deciding as to the affinities presented 

 by its larval stages to certain of the Trilobita, such relations 

 being only in general external form. Dr. Packard (Reports of 

 the American Association for the Advancement of Science, Au- 

 gust, 1870) remarks, '' The whole embryo bears a very near 

 resemblance to certain genera of Trilobites, as Triniideus, Asa- 

 phiis, and others;" and he adds, " previous to hatching it strik- 

 ingly resembles Triniideus and other Trilobites, suiriiestins: that 

 the two groups should, on embryonic and structural grounds, be 

 included in the same order, especially now that Mr. E. Billings 



* One in Canada, and cue in the British Museum, hoth of the 

 same species. 



