HEMIASPIS LIMULOIDES. 



177 



structure. Dr. J. Nieszkowski has described two forms from the Upper Silurian of the 

 Island of Oesel, namely, Pseudoniscus aculeatus (Woodcut, Fig. 65), and Exapinurus 

 Schrenkii (Woodcut, Fig. 66) ; and Dr. d'Eichwald has described a third form under 

 the title of Bunodes lunula (Woodcut, Fig. 67), from the same rich locality and 

 formation. 1 



Pig. 64. — Semiaspis limuloides, H. 



Woodward, Lower Ludlow, Leint- 



wardine. 



h, the head; th, the six thoracic 



segments; ab, the three abdominal 



somites; t, the telson. 



Fig. 65. — Pseudoniscus aculeatus, 

 Nieszk., Upper Silurian, Isle of 

 Oesel, Baltic. 



Fig. 66. — Exapinurus Schrenkii, 

 Nieszk., U. Silurian, I. of Oesel, 

 Baltic. 



Fig. 67. — Bunodes lunula, 2 

 Eichw., U. Silurian, I. of 

 Oesel, Baltic. 



All these fo»ms have the three well-marked divisions to their bodies into head, thorax, 

 and abdomen, and all, save Bunodes, possessed a telson, or tail-spine, and free articulated 

 thoracic somites. 



In addition to Hemiaspis limuloides, already described, there are certain other specimens 

 in the Museum of Practical Geology, to which Mr. Salter has appended MS. names, 

 namely — 



Hemiaspis {Limuloides) speratus, Salter MS. 

 „ ,, op tat us, ,, 



„ „ ttiberculatus, „ 



1 'Archiv fur die Naturk. Liv- Ehst- und Kurlands,' erste Series, vol. ii, pi. ii, figs. 12, 13, and 15, 

 pp. 378—382, Dorpat, 1859, 8vo. 



2 It is just possible that Bunodes may prove to be an Arachnid related to Scudder's Architarbus 

 rotundatus from Illinois, U.S. 



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