46 



Mr. J. W. Salter on the occurrence of a Fish 



ginate; the posterior end (not perfect in fig ], but nearly so in 

 the Upper Ludlow specimen*) has an obtuse short keel, but is 



Fig. 1. Fig- 2. 



P. ludensis. 



P. truncatus. 



Outline-diagrams of Pteraspis. The figures are not strict drawings, but intended 

 to point out the distinguishing characters of the two species. 



not prolonged, to all appearance, into a spine, as in P. truncatus ; 

 nor is the posterior margin nearly so oblique. The fragment is 

 sufficiently perfect to show this. The lateral angle a, which 

 terminates the bevelled sutural edge Z>, is further back in the 

 new species. 



Besides the narrower form, there is a decided difference in 

 the direction of the sculpturing. In P. truncatus (fig. 2) the 

 striae and also the stronger intermediate ribs converge posteriorly 

 towards the ridge and spine, and again anteriorly towards an 

 obscure median line. In this species, on the other hand, they 

 start more parallel from the posterior margin, and near the 

 anterior end diverge towards the outer angles. So far as the 

 preservation of the specimen permits of judging, there are no 

 stronger striae anteriorly. 



As there seem to be sufficient characters to distinguish the 

 species, it will be well not to confound it with P. truncatus, to 

 which it was doubtfully referred in the second edition of ^ Silu- 

 riat«' It may be called 



P. ludensis, n. sp. 



P. scute nuchali elongato, oblongo-ovali, convexo nee gibbo, antice 

 truncate etiam emarginato, postice brevicarinato baud longispi- 



* It is added in dotted outline to fig. 1. 



t Page 267- It is mentioned as probably distinct, in the note on Pter- 

 aspis appended to Mr. R. Banks's paper, before quoted. This referred only 

 to the Upper Ludlow specimen with an incomplete surface. 



