﻿ACTINOPTERIA. 



75 



?18S1. Avicula (Pterinea ?) migbans, Barrande. Syst. Sil. Boheme, vol. vi, 



pi. ccxxix, fig. 12, 1-12 a, Et. P. 

 1S88. Pterinea texturata, Etheridge. Foss. Brit., vol. i, Pal., p. 159. 



Description. — Left valve large, sub -quadrate, moderately transverse, convex. 

 Umbo prominent, rounded, proximate, arching obliquely forward, extending 

 slightly above the hinge-line, and situated at the anterior point of the shell, 

 excluding the anterior wing. Hinge-line straight, equal to the extreme length of 

 the shell, apparently rather oblique in front of the umbo, and bearing a slight 

 linear ligamental (?) groove from the umbo to the hind margin. Anterior wing large, 

 obtusely triangular, convex, separated by a broad concavity from the body of the 

 shell. Anterior margin oblique, concave in the centre, convex above and below. 

 Inferior margin wide, slightly oblique and convex. Postero-inferior corner rather 

 produced and roundly convex. Posterior side oblique. Surface covered with 

 numerous, coarse, distant, elevated ribs, tesselated by coarse, distant, regular, 

 elevated threads or laminae. 



Size of left valve. — Length 25 mm., breadth 20 mm., depth 5 mm. Another 

 specimen is 37 mm. in breadth and 14 mm. in depth. 



Localities. — From Wolborough there are two specimens of the left valve in 

 Mr. Vicary's Collection, two in the Museum of Practical Geology, and two, 

 perhaps from the same locality, in the Torquay Museum. From Lummaton there 

 is one specimen in Mr. Lee's Collection, one in the Woodwardian Museum, two 

 doubtful specimens in my Collection, and five, probably from the same locality, 

 in the Torquay Museum. A right valve from Lummaton in my Collection perhaps 

 belongs to this species. 



Remarks. — From the fragmentary condition of our specimens and the amount 

 of variation displayed in them it is not very easy to be sure of their specific limits, 

 nor is it absolutely certain that they are really more than the coarser variety of 

 one of the adjoining forms. On the other hand, it seems to me that the presump- 

 tion is that they will ultimately be found to constitute one well-defined species, 

 and that there is no doubt that they belong to the form to which Phillips has 

 given the name Avicula texturata. 



This form seems characterised by its large, convex, confluent front wing, its 

 coarse ribs barred with strong sharp concentric strias, and its oblique and rather 

 short anterior side. 



There are several Bohemian shells which present considerable resemblance to 

 it. Of these Avicula migrans, Barrande, is probably identical, as it only differs in 

 some slight particulars, e. g. in having the anterior side rather narrower, and the 

 posterior margin rather more concave. 



I am doubtful whether the right valve in my Collection should be referred to 

 this species or to A. dilatata. It has about thirty strong, sharp, distant threads, 



