230 



BRITISH FOSSIL CRUSTACEA. 



II. 



(ElJRYPTERIDA.) 





(XlPHOSURA.) 



Thorax : — Bearing the 





II. Thorax: — Bearing the 



[l. 1 ] Opercular plate having 



the ovarian 



[l. 1 ] Opercular plate having the ovarian 



openings on its inner s 



urface. 



openings on its inner surface. 



2. Branchiae. 





2. Branchial feet. 



3. (Ditto?.) 





3. Ditto. 



4. No appendages. 





4. Ditto. 



5. Ditto. 





5. Ditto. 



6. Ditto. 





6. Ditto. 



7. Ditto. 





7. No appendages. 



Abdomen (well developed). 





III. Abdomen (rudimentary). 



No appendages. 





No appendages. 



1 — 7. Including the telson. 





1, 2. Segments and telson. 2 



III. 



The eminent American carcinologist, Professor Dana, writing on Limulus, says, " The 

 body consists of three segments, and they may be compared to the segments in Caligus. 

 The anterior segment bears six pairs of members ; the first appears to correspond to the 

 second pair of antennae (or third normal segment), the 2nd, 3rd, 4th, 5th, and 6th to the 

 mandibles and the four following pairs of members (or the fourth to the eighth normal 

 segment inclusive). 



" The second segment of the body, which we consider as a continuation of the 

 cephalothorax, and not abdominal, bears six pairs of foliaceous organs, analogous to the 

 foliaceous appendages of the posterior part of the thorax in certain Caligida, in some of 

 which one or two pairs of legs are combined into a hard, thin plate like an apron. These 

 six pairs make up exactly the normal number of cephalothoracic segments, namely, 

 fourteen. The abdomen, according to this view, is confined to the last or third segment," 

 i. e. the great telson and any portion of the hind-body, the segments of which, though 

 not seen in the adult, are indicated by distinct sutures in the young (see PI. XXXIII, 

 fig. 12, A and t). 



In this sense I shall be happy to adopt the term cephalothorax, but not as applied by 

 Dr. Packard to the head-shield alone. 



6th. As to the suggestion that my division between the thorax'and abdomen in the 

 Eurypteuida " is a purely imaginary one," I will only refer those interested in the 

 subject to the figures of Stylonurus (p. 131, fig. 39, dorsal view), in which, as in many 

 other cases, the ornamentation ceases at the end of the thoracic series; to the figure of 

 Eurypterus (p. 132, fig. 40), and to the still more striking figure of Hemiaspis (p. 177, 

 fig. 64). 



7th and lastly. In the method adopted by me in counting the segments I have been 

 actuated by the principle long since suggested by Oken, that, even where a segment is 



1 Both Professors Huxley and Owen have demonstrated that the nerves supplying this opercular plate 

 arise from the cephalic shield. 



2 Prof. Owen has shown that the ' telson ' most probably represents the undeveloped posterior 

 abdominal somites as evidenced by the plexus of nerves it encloses within its hard triangular sheath. 



