ARTHROPLEURA MAMMATA. 165 



across. It shows a strongly arched border running out into a short acuminate broad spine, 

 into which a strongly curved sharpish ridge runs from about the upper central portion. 

 The space outside and above the ridge is flat, and is ornamented with small rounded 

 tubercles scattered irregularly over its surface ; below the ridge there are fewer tubercles, 

 but near the hinder border there are two large mammillated tubercular spines (and part of a 

 third) ranged about equidistant from one another ; they are directed backwards, and 

 are fully a third of an inch in length, and as much in diameter at the swollen base. 



Another fragment (PI. XXIV, fig. 3), also referred by Mr. Salter to the head, shows 

 large and small tubercles upon its surface. 



Mr. Salter speaks of " at least six, and probably more, of the large mammillated tuber- 

 cular spines along the hinder border" (of fig. 2, PI. XXIV). The explanation of this is 

 to be found by referring to the woodcut illustrating Mr. Salter's paper, which by the kind- 

 ness of the Council of the Geological Society we are able to give above, p. 164, Fig. 54 

 (see 'Quart. Journ. Geol. Soc./ vol. xix, p. 84), when it will be seen that this specimen 

 (fig. 2) has been drawn twice over (once reversed, and once in its natural position) so as 

 to give the effect of a crescent-shaped shield. But only the piece represented on our Plate 

 as fig. 2 1 exists, and after careful comparison I am quite unable to refer it to the head- 

 shield of any known Crustacean. 



Referring to the above woodcut Mr. Salter observes, " Although I have supposed 

 figs. 1 and 2 to belong to the head, I have really little else to recommend this view than 

 the great comparative size and breadth, and the general form, which is like that of the hinder 

 angles of the head of the Scotch Eurypterus." 2 



Mr. Salter then proceeds to contrast his Eurypterus mammatus with the Arthropleura 

 armata of Jordan, a species founded upon the fragments of a large Crustacean from the 

 Coal-measures of Saarbruck, Rhenish Prussia. 



If the subjoined woodcut (Fig. 55, b) of Arthropleura armata be compared with PI. 

 XXIV, fig. 2, a somewhat similar piece (marked p), having a corresponding raised border 

 and similar arrangement of tubercles, will be seen, suggesting that this pointed portion 

 (fig. 2) may more likely be the pleuron of a trilobed segment. This interpretation, 

 although rejected by Mr. Salter, seems to me, on many accounts, more probable than the 

 alternative one which he has adopted, namely, that of supposing it to be the posterior 

 angle of the head-shield of Eurypterus, with which indeed it seems exceedingly difficult 

 to associate it. 



1 It is quite apparent, however, from Mr. Salter's description, that he was under the impression that two 

 specimens existed in reality as well as in his figure. I have therefore specially noticed this, lest hereafter it 

 should be supposed that one of these curious fragments had been lost. 



2 If by the Scotch Eurypterus Mr. Salter refers to E. Scouleri, a comparison between the fragment of 

 E. mammatus (PI. XXIV, fig. 2) and the entire head of E. Scouleri (PI. XXVII) will satisfy us that such a 

 hinder angle could not possibly be fitted to the carapace of the latter ; the curvature and ornamentation 

 being both incongruous. No other form with which we are acquainted could have required so large an 

 epimeral piece save the great Devonian Stylonurus Scoticus. 



