A 2^ew Faintly and Genua of Lower Silurian. Crustacea. 165 



inal median projections of their sterna, which point outwards when 

 the abdomen is flexed upward. Evidences of the tergal, pleural and 

 sternal sutures of the abdominal segments remain. Lateral append- 

 ages, and anterior portion of body unknown. 



Among the palaeozoic representatives of the anomalous class to 

 which our fossil belongs, those of most interest to us in this connec- 

 tion are the Trilohita and Eurypterida^ forming, as they do, the most 

 important evidence relating to the extinct Crustacea. It is not cer- 

 tain that either antennae or feet have been found with the remains of 

 the former, nor is anything known of other than the tergal and pleural 

 portions of the somites. No sternal element or body-wall has been 

 found, and no certain evidence of any appendages. The thoracic 

 somites have the cuticular portion of the pleura folded inward, so that, 

 in well-preserved specimens, it maj' be seen to cover a portion of their 

 sternal surface. But as this haixlened inner portion of the pleura never 

 extends to the sternal region, and as no sternal arch is ever found, it is 

 probable that this portion of the somites was perishable. In this 

 aspect of the case, as well as in most others, the Trilohita threw no 

 light on the affinities of the fossil herein described. The case is some- 

 what different with the Eurypterida. These giants of the Merostom- 

 ata offer some suggestive facts which will now be briefly considered. 

 The appendages, both in Pteryjotus and Eurypterus.^ are carried by a 

 comparatively short cephalo-thorax. This region of the body is fol- 

 lowed by a large number (twelve or thirteen of abdominal segments) 

 terminated b}^ a long telson. None ol these bear any appendages. A 

 glance at any specimen or good figure of these genera, will show that if 

 a comparativel}^ small portion of the bod}^ were lost anteriorly, we 

 should be left entireh" to conjecture as to the nature of all the append- 

 ages, and it seems not unlikely that our fossil may be characterized by 

 the same peculiarity. At all events, as these fossil Crustaceans com- 

 bined characters belonging both to the Ilerostomata, and to the Cope- 

 poda among the Entowosfraca, it is not unreasonable to suppose that 

 Enoploura may have been equally indefinite in phylogenic relation. 

 If, as seems clear from the investigations of Mr. Spence Bate and 

 others, the normal number of segments in the typical Crustacean is 

 twenty, we have but four somites of the body in our species as yet un- 

 described; but it must not be forgotten that among the Entomostraca, 

 the number of somites may be greater or less than this, while the 

 Branchiopoda.^ closely united to the lower Podojihthalmia. through 

 such forms as Jfysis, have also a greater or less number of somites 

 than the typical Crustacean; thus Nehalia has twent\-two, and Ap)us 

 glacialis twent3'-six, in the thoracico-abdominal region alone. Con- 



