PTERINOPECTEN RIGIDUS. 61 



down the shell. The whole of the radiating ribs are crossed at intervals by more 

 or less well-marked concentric lines of growth, which may be strong enough to 

 interrupt the ribs. This condition is best marked on both ears. 



Dimensions. — PI. VIII, fig. 3, as restored, measures — 



Antero-posteriorly . . . .85 mm. 



Dorso-ventrally . . . .75 mm. 



Convexity of left valve . . .10 mm. 



Localities. — Ireland : co. Fermanagh, in Shales at Enniskillen ; Lisnapaste, 

 Ballintra, co. Donegal; Millicent, co. Kildare. 



Observations. — It is difficult to understand why Portlock referred this species to 

 flli is, as the Aviculoid character of the shell is very apparent, and it possesses no 

 Brachiopod-like character. The type specimen is exceedingly valuable, and is 

 preserved in the Museum of the Geological Survey, Jermyn Street. It is a 

 specimen of a left A 7 alve (PI. VIII, fig. 3), entirely free from matrix, but has lost 

 some half -inch of the lower part of its circumference and the extremities of the 

 ears. Its hinge-line, however, is intact, and a view of this is given in PL VIII, 

 fig. da. This hinge-line recalls very forcibly the hinge-plates of Myalina and 

 Naiadites, and to a certain extent of Leiopteria and Pterinea. Hall figures a 

 specimen of Lyriopecten (a genus which is characterised by a hinge-line of a length 

 intermediate between that of Pterinopecten and Aviculopecten), L. orbiculatus, from 

 the Upper Helderberg and Hamilton Groups of New York, and possessing a 

 similar hinge-plate to that shown in P. rigidus (' Pal. New York, Lamelli- 

 branchiata,' vol. i, pi. iv, figs. 7 and 9). The shape of P. rigidus shows its strong 

 resemblance to recent forms of Meleagrina, which possess a hinge-plate differing 

 only slightly from their Palaeozoic congener. 



The type of Meleagrina rigida, M'Coy, is unfortunately not in the Griffith 

 Collection of the Museum of Science and Art, Dublin, but I have no hesitation in 

 referring Portlock's shell to M'Coy's species. M'Coy's description of the ribs is as 

 follows: — " Surface with sixty-five rather distant, narrow, rough, radiating ridges, 

 crossed by a few large irregular concentric wrinkles. . . . The radiating ribs 

 are narrow, rough, and separated by broad Hat spaces." His figure is a good one, 

 and evidently drawn from a perfect specimen. It is much to be regretted that the 

 specimen has disappeared. Tt seems to me not unlikely that Malleus orbicularis, 

 M'Coy, may be the right valve of /'. rigidus. Unfortunately the type has dis- 

 appeared, and 1 have met with no other shell like it. M. orbicularis was founded 

 on a flat right valve, with a much-produced ami pointed posterior wing, very 

 similar in shape to that of /'. rigidus, the right valve of which has never been 

 described. 



I refer the original of PI. VIII, tig. 1 , wit h hesitation to the species, on account 

 of its small posterior wing; but noting the lines of growth on tin- specimen shown 



