1868.] 



WOODWAKD SILURIAN CRUSTACEA. 



295 



To add to the difficulty, a third form, readily separable from 

 Pteriigotus h'dohus and Ptenjijotus perornatus by the greater breadth 

 and narrowness of the thoracic segments in proj^ortion to the suc- 

 ceeding abdominal ones, has been found, also possessing a bilobed 

 telson. 



I see no other way towards a solution of this difficulty, except by 

 suggesting that the name hilohus be retained for all the three forms 

 with bilobed non-acuminate telsons, treating them, for the present, 

 as varieties only, until we obtain more positive evidence of their 

 specific distinctness than we at present possess. The name hilohus 

 may then be disused and the varietal appellation become of specific 

 value. I propose, then, to term them thus : — 



a. Fterygotus hilohus, var. inornatus. 

 /3. ,f „ var. perornatus. 



y. „ „ var. crassus. 



The following Table, giving the measurements of two specimens 

 of each variety, will furnish an idea of the relative proportions of 

 the three forms : — 





Head. 



Thorax. 



Abdomen. 



Telson. 



Lat. 



Long. 



Lat. 



Long. 



Lat. 



Long. 



Tiat. 



Long. 





in. lin. 



in. lin. 



in. lin. 



in. lin. 



in. lin. 



in. lin. 



in. lin. 



in. lin. 



Var. inornatus... 



1 17 



13 



1 9 



2 3 



1 



2 6 



8 



11 





1 16 



12 



1 9 



1 4 



1 



2 



7 



11 



,, perornatus. 



3 



22 



3 9 



4 



2 



4 6 



17 



1 9 



J) >> • • • 



2 6 



2 3 



3 9 



4 6 



2 



4 9 







„ crassus 



1 8 



13 



2 



1 3 



1 



1 6 



9 



1 



„ •- 



2 



1 6 



2 6 



1 6 



1 3 



2 3 



13 



1 3 



N.B. The breadth given is the greatest breadth, and the length also at the 

 longest point of the head, &c. 



EXPLAJ^ATION OF PLATES IX. & X. 



(Illustrative of Silurian Crustacea.) 



Plate IX. 



Fig. la. Eurypterus scorpioides, Salter, MS. Nearly perfect specimen (two- fifths 

 the natural size) from tbe Uppermost Ludlow Eock, Lesmabagow, Lanark- 

 shire. Tbis specimen exhibits tbe dorsal aspect, a, a. Imperfect antennas. 

 h, b. Palpi with long recurved spines, e, e. Ectognaths, or swimming-feet. 



Fig. \h. Portion of one of tbe thoracic segments, of tbe natural size, to show the 

 punctate ornamentation. 



Fig. 2. Metastoma or postoral plate referred to Eurypterus punctatus, Salter 

 (one-third natural size), from the Lower Ludlow rock, Leintwardine, 

 Shropshire. 



Fig. 3. Fterygotus raniceps, H. Woodw. (twice the natural size), from the Upper- 

 most Silurian, Lanarkshire. 



Drawn from specimens in the British Museum. 



Plate X. 



Fig. 1. Eurypterus obesus, H. Woodw. (twice the natm-al size). Original spe- 

 cimen in the British Museum. 

 VOL. XXIV. PART I, Y 



