60 CARBONIFEROUS LAMELLIBRANCHIATA. 



swelling the shell becomes compressed rapidly into the hinge-line. There appears 

 to be no constriction anterior to the swelling, nor any indication of a byssal sinus in 

 the type specimen, which has the test preserved, but this character is markedly 

 present in casts. 



Interior. — The anterioi' adductor scar is almost marginal, fairly large and round, 

 quite anterior to the umbo. It is bounded behind in casts by a deep groove which 

 passed downwards and obliquely backwards to the lower border, becoming shallower 

 and broader en route. In casts the umbo is separated from the hinge-line by a 

 distinct hollow, which expands and becomes shallower as it passes backwards. 

 The interior of the umbo was smooth and acutely pointed. The positions of 

 the posterior adductor and the pedal muscle-scars have not yet been exposed. 



There is no groove along the hinge-line as in casts of Anthracomya, nor area for 

 the internal ligament as in recent Modiolte. 



Exterior.- — The surface is marked with fine raised thread-like striae, fresh ones 

 arising over the greatest gibbosity of the shell by bifurcation or intercalation. 

 Posteriorly they are fewer, less raised, and farther apart, and often joining another. 

 The direction of the stride is parallel to the margins, and they all terminate in the 

 hinge-line. On the flattened and compressed part of the shell above, and posterior 

 to the umbonal swelling, the surface is almost smooth. The shell is thin. 



Dimensions. — The type of M. Macadamii, var. elongata, PI. II, fig. 26, measures — 

 Greatest antero-posterior diameter . . 30 mm. 



Greatest dorso-ventral ,, . .18 mm. 



The type of M. Macadamii, var. lata, PI. II, fig. 25, measures- 

 Greatest antero-posterior diameter . . 32 mm. 

 Greatest dorso-ventral ,, . .20 mm. 



Localities. — Derry, Ballynascreen, Tyrone ; LoAver Limestone-shales, Cultra, 

 CO. DoAvn. 



Observations. — Owing to the preoccupation of the name Modiola elongata by 

 Professor Phillips, though that shell is a Posidoniella, I have been unable to 

 retain it as the specific title of Portlock's shell. I am, however, not able to find 

 any specific characters on which to separate Portlock's variety of Modiola Macadamii, 

 var. elongata, from his var. lata, and have therefore retained that name for both 

 shells. The apparent differences between the two types are due to the fact that the 

 var. elongata possessed a Avell-preserved test ; Avhile var. lata, being a somewhat large 

 shell, Avas mainly a cast, but unfortunately did not give much detail as to the internal 

 anatomy ; this want, however, was fortunately su23plied by another specimen in 

 the Geological Survey Collection, PI. II, fig. 27, which shoAved many of the internal 

 characters. One curious feature is the deep byssal groove in casts, all evidence of 

 which is absent in the complete shell. This is shown to be due to an oblique ridge 

 on the inner surface of the shell in fig. 23, PI. II, from the Culti\a beds. Portlock 



