Senry Woodtoard — On Fossil Crustacea. 523 



that the Trilobita were an earlier and more generalized type of Crustacea from -whicli 

 the later and more specialized Isopoda have arisen. 



The large compound sessile eyes, and the hard, shelly, many-segmented body, with 

 its compound caudal and head-shield, differ from any known Phyllopod, hut offer 

 many points of analogy with the modern Isopods, and one would be led to pre- 

 suppose the Trilobites possessed of organs of locomotion of a stronger texture than 

 mere branchial frills. 



The objection raised by Drs. Dana and Verrill to the special case of appendages in 

 the Asaphus 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 suggested, between the 

 abdomen of a Maerouran Decapod and the Trilobite in question, is the best refata- 

 tion 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 cor- 

 respond to these semicalcified arches in the Maerouran 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 {not 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 

 branchiae. In the case of the Trenton Asaphus I shall be satisfied if it appears, from 

 the arguments I have put forward, that they are most probably 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. ^ 



"With regard to the embryology and development of the modern King-Crab 

 {Limulus polyphcemus), 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, August, 1870) remarks, " The 

 whole embryo bears a very near resemblance to certain genera of Trilobites, as 

 Trinucleus, Asaphus, and others;" and he adds, "previous to hatching it strikingly 

 resembles Trinucleus and other Trilobites, suggesting that the two groups should, on 

 embryonic and structural grounds, be included in the same order, especially now that 

 Mr. E. Billings has demonstrated that Asaphus possessed eight pairs of five-jointed 

 legs of uniform size." 



Such statements are apt to mislead, unless we carefully compare the characters of 

 each group. And first let me express a caution against the too hasty construction of 

 a classification based upon larval characters. 



Larval characters are useful guide-posts in defining great groups, and also in indi- 

 cating affinities between great groups ; but the more we become acquainted with 

 larval forms the greater will be our tendency (if we attempt to base our classification 

 on their study) to merge groups together which we had before held to be distinct. 



To take a familiar instance : if we compare the larval stages of the Common 

 Shore-Crab {Garcinus moenas) with Fterygotus, we should be obliged (according to 

 the arguments of Dr. Packard) to place them near to or in the same group. 



The eyes in both are sessile, the functions of locomotion, prehension, and mastica- 

 tion are all performed by one set of appendages, which are attached to the mouth ; 

 the abdominal segments in both are natatory, but destitute of any appendages. 



Such characters, however, are common to the larvae of many Crustaceans widely 

 separated when adult, the fact being that in the larval stage we find in this group, 

 what has been so often observed by naturalists in other groups of the animal kingdom, 

 namely, a shadowing forth in the larval stages of the road along which its ancestors 

 travelled ere they arrived from the remote past at the living present. 



If we place the characters of Limulus and Fterygotus, side by side, and also those 



1 Oue in Canada, and one in the British Museum, both of the same species. 



