MEROSTOMATA. 1 19 



6. There is nothing among the trilobites comparable to the movable lateral spines of 

 the metasoma of Limulus. 



While, as classifications are made up, the Trilobita must be placed in the Crustacea 

 rather than the Arachnida, there is no reason why both the modern Crustacea and the Arach- 

 nida should not be derived from the trilobites. 



MEROSTOMATA. 



It has been a custom of long standing to compare the trilobite with Limulus. Packard 

 (1872) gave great vitality to the theory of the close affinity of the two when he described 

 the so called trilobite-stage in the development of Limulus polyphemus. His influence on 

 Walcott's ideas (1881) is obvious. Lankester has gone still further, and associated the 

 Trilobita with the Merostomata in the Arachnida. 



The absence of antennules at any stage in development allies Limulus so closely with 

 the Arachnida and separates it so far from the Trilobita that in recent years there has been 

 a tendency to give up the attempt to prove a relationship between the merostomes and trilo- 

 bites, especially since Clarke and Ruedemann, in their extensive study of the Eurypterida, 

 found nothing to indicate the crustacean nature of that group. A new point of view is, how- 

 ever, presented by the curious Sidneyia inexpectans and Emeraldella brocki described by 

 Walcott from the Middle Cambrian. 



Sidneyia inexpectans Walcott. 



Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1911, p. 21, pi. 2, fig. I (not figs. 2, 3); pis. 3-5; 

 pi. 6, fig. 3; pi. 7, fig. i. 



The body of this animal is elongate, somewhat eurypterid-like, but with a broad telson 

 supplied with lateral swimmerets. The cephalon is short, with lateral compound eyes. The 

 trunk consists of eleven segments, the anterior nine of which are conspicuously wider than 

 the two behind them, and the telson consists of a single elongate plate. 



On the ventral side of the head there is a large hypostoma and five, pairs of appendages. 

 The first pair are multisegmented antennules. The second pair have not been adequately 

 described. The third are large, complex claws, and the fourth and fifth suggest broad, 

 stocky endopodites. Broad gnathobases are attached to the coxopodites of the third to fifth 

 pairs of appendages and form very strong jaws. 



The first nine segments of the thorax have one pair each of broad filiform branchial 

 appendages, suggestive of the exopodites of trilobites, but no endopodites have been seen. 

 The tenth and eleventh segments seem to lack appendages entirely. 



Emeraldella brocki Walcott. 



Illustrated : Sidneyia inexpectans Walcott partim, Smithson. Misc. Coll., vol. 57, 1911, pi. 2, figs. 2, 3 

 (not fig. i) ; Ibid., 1912, p. 206, text fig. 10. 



Emeraldella brocki Walcott, Ibid., 1912, p. 203, pi. 30, fig. 2; text fig. 8; Ibid., vol. 67, 1918, p. 118 

 (correction). 



Emeraldella has much the same shape as Sidneyia and the same number of segments, 

 but instead of a broad flat telson, it has a long Limulus-\ike spine. The cephalon is about 

 as wide as long, and eyes have not yet been seen. The body consists of eleven segments and 

 a telson (Walcott says twelve and a telson but shows only eleven in the figures). Nine of 

 the segments, as in Sidneyia, are broad, the next two narrow. 



