I2O THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



The ventral side of the cephalon has a long hypostoma, and five pairs of appendages. 

 The first pair are very long multi segmented antennules and the next four pairs seem to be 

 rather slender, spiniferous, jointed endopodites. Whether or not gnathobases were present 

 is not shown by the figures, but owing to the long hypostoma the appendages are grouped 

 about the mouth. All the segments of the body, unless it were the telson, seem to have 

 borne appendages. On the anterior end, they were clearly biramous (1912, p. 206, text 

 fig. 10), and that they were present along the body is shown by figure 2, plate 30, 1912. 



The present state of knowledge of both these peculiar animals leaves much to be desired. 

 The indications are that the cephalic appendages are not biramous, and that only one pair of 

 antennae, the first, are developed as tactile organs. The thoracic appendages of Emcraldclla 

 are biramous, and also possibly those of Sidneyia. In the latter, the last two abdominal seg- 

 ments seem to have been without appendages, while in Emcraldella at least one branch of 

 each appendage, and possibly both, is retained. 



These animals, which may be looked upon as the last survivors of an order of pre- 

 Cambrian arthropods, have the appearance of an eurypterid, but their dominant character- 

 istics are crustacean. The features which suggest the Eurypterida are : elongate, obovate, 

 non-trilobate, tapering body; telson-like posterior segment; marginal, compound, sessile eyes; 

 claw-like third cephalic appendages ; and, more particularly, the general resemblance of the 

 test to that of an eurypterid like Strabops. In form, Sidneyia agrees with the theoretical 

 prototype of the Eurypterida reconstructed by Clarke and Ruedemann (Mem. 14, N. Y. 

 State Mus., vol. i, 1912, p. 124) in its short wide head with marginal eyes, and its undiffer- 

 entiated body. There is, moreover, no differentiation of the postcephalic appendages. 



The crustacean characteristics are seen in the presence of five, instead of six, pairs of 

 appendages on the head, the first of which are multisegmented antennules, and in the bira- 

 mous appendages on the body of Emeraldella. It should be noted that these latter are 

 typically trilobitic, each consisting of an endopodite with six segments and a setiferous 

 exopodite. 



Clarke and Ruedemann (1912, p. 406) have discussed Sidneyia briefly, and conclude: 



It seems to us probable that the Limulava [Sidneyia and Amiella] as described are not eurypterids but 

 constitute a primitive order, though exhibiting some remarkable adaptive features. This order possibly 

 belongs to the Merostomata, but is distinctly allied to the crustaceans in such important characters as the 

 structure of the legs and telson, and is therefore much generalized. 



The specialization of Sidneyia consists in the remarkable development of a highly com- 

 plex claw on each of the third cephalic appendages, and in the compound tail-fin, built up 

 of the last segment and one or more pairs of swimmerets. These two characteristics seem 

 to preclude the possibility of deriving the eurypterids from Sidneyia itself, but it seems 

 entirely within reason that they may have been derived from another slightly less specialized 

 member of the same order. 



That Sidneyia is descended from any known trilobite seems highly improbable, but that 

 it was descended from the same ancestral stock as the trilobites is, I believe, indicated by 

 the presence of five pairs of appendages on the cephalon and trilobitic legs on the abdomen. 



Molaria and Habelia. 



Other so called Merostomata found by Walcott in the Middle Cambrian are the genera 

 Molaria and Habelia, both referred to the Cambrian family Aglaspidae. These genera seem 

 to conform with Aglaspis of the Upper Cambrian in having a trilobite-like cephalon without 



