PARALLELODON DIVISUS. 173 



posterior-inferior angle, becoming gradually less and less conspicuous and finally 

 obsolete. Above the ridge the shell is compressed and expanded so as to become 

 subalate, and the dorsal slope is hollow. The general convexity of the valves is 

 interrupted by a pronounced oblique byssal groove, which starts from the umbo 

 above, and becoming deeper and wider as it crosses the valve, descends obliquely 

 backwards and downwards to the inferior border. 



Interior. — The adductor muscle-scars are not apparent. The hinge consists 

 of a few small, microscopic oblique teeth in front, and three to four elongate 

 subparallel teeth behind. 



Exterior. — The surface is adorned with fine, regular, subimbricating, concentric 

 lines, which are more widely separated on the dorsal slope where they follow the 

 concavo-convex contour of the border, and near the inferior border they are 

 markedly interrupted by the byssal groove. 

 Dimensions. — Fig. 11, PI. XIII, measures — 



Antero-posteriorly . . . .12 mm. 



Dorso-ventrally . . . .6 mm. 



Localities. — Scotland : In a bed of Limestone east of the Caves of Randerstone 

 Castle, Calciferous Sandstone series, Fife; Scorpion Beds, near Langholm, 

 Eskdale, Dumfriesshire ; Cyprid Shale, Colinton Road, Craiglockhart Hill, and 

 "Woodhall, Water of Leith, near Edinburgh, all in the Cement Stone group, 

 Calciferous Sandstone series. Ireland : Larganmore, Bangor. 



Observations. — Described as Modiola by M'Coy, and much later as Avicula by 

 R. Etheridge, jun. D'Orbigny as long ago as 1850 correctly recognised the true 

 affinities of the species to Area. The discovery of the hinge, several examples of 

 which I have been fortunate enough to expose on a slab from Randerstone, Fife, 

 given me by Mr. J. "W. Kirkby, of Leven, shows that this view was substantially 

 correct. 



The characteristics of this species are so marked that there can be no doubt 

 as to its identity, no other species of Parallelodon, so far as I am aware, possessing 

 the marked byssal groove or peculiarly cut posterior border. 



M'Coy's specimen does not seem to have shown this characteristic of the 

 posterior margin, what is probably due to the imperfect cleaning of his specimen, 

 which I reproduce (PI. XIII, fig. 13). M'Coy's description of Modiola divisa is as 

 follows {op. supra cit.) : " Transversely ovate, twice as wide as long, gibbous, 

 beaks tumid, close to the anterior end ; hinge-margin elevated, rectangular, 

 as long as the shell is wide, anterior end short, rounded ; posterior end subtruncate, 

 rounded ; a small notch or sinus in the abdominal margin, from whence a deep 

 sulcus extends entirely to the beak; surface very finely wrinkled transversely." 

 The type specimen is on a small block with numerous examples, some of which 

 are casts, and show a longitudinal groove parallel to the hinge-plate behind. 



