64 CARBONIFEROUS LAMELLIBRANCHIATA. 



Dimensions. — The specimen marked as the type, PI. II, fig. 20, though im- 

 j^erfect, measures — 



Greatest antero-posterior length . . .13 mm. 



Greatest dorso-lateral length . . .5 mm. 



Localities. — Ireland : Ballynascreen, co. Derry ; Tyrone. 



Observations. — In the original description of the species Modiola Macadamii {op. 

 supra cit.) General Portlock gave names to three varieties, without enunciating 

 the specific characters or types. He states, p. 432, "The three forms figured 

 . seem in themselves sufficiently distinct for specific separation, but there 

 are so many intermediate gradations, and so much variation from age and the 

 condition of the specimens that I shall unite them together as merely varieties of 

 one s^Decies." In his remarks on M. Macadamii, var. angusta, he says, " This 

 diverges as much from the ordinary form in one direction as fig. 15 [M. Macadamii, 

 var. lata) does in another ; " and this makes it appear that he regarded neither 

 of the three varieties as the specific type. This omission is unfortunate, as no 

 description or figure exists of such, and therefore it has been necessary to regard 

 Portlock's variety angusta as the type, this form being the first one of the three 

 on page 432 of Portlock's work. The block on which the figured specimen of M. 

 Macadamii, var. angusta, occurs is of totally different lithological character, and 

 contains a different fauna from those in which the other described varieties were 

 found, — the first-named shell occurring with a typically marine fauna, containing in 

 abundance Nucidana {Leda) attenuata, Avicula Hendersoni ? R. Etheridge, jun. 

 [Modiola divisa, M'Coy), and Ostracoda ; while with the other varieties only some 

 other forms of Ostracoda are found. In the Explanatory Memoir of Sheet 25 of 

 the Geological Survey, p. 22, the shells are said to occur with Spirorbis, Helodus, 

 Lejyerditia Okeni, and plant remains ; and in two others with Ortlioceras, 

 Euomphalus Cyrtoceras, and Syringopora. 



I am of opinion that this form is specifically distinct from the other two ; both from 

 the general appearance and the associated fossils, which indicate a more truly 

 marine condition. From the fact that in most of the localities the shells were of 

 estuarine or brackish-water habitat, it seems to me probable that the vars. lata and 

 ehngata were associated with them ; and that the form angusta only occurred in the 

 true marine beds. 



I have therefore retained the name Modiola Macadamii for the variety angusta, 

 and shall describe the other forms under the names Modiola lata, owing to a pre- 

 occupation of the name M. elongata by Professor Phillips (' Geol. Yorkshire,' part 2) : 

 I can see no essential difference between them. 



Immediately after the description of the three forms of M. Macadamii, Portlock 

 described another form as Modiola subparallela, on which he remarks, " Though this 



