THE LAMELLIBRANCHS OF THE SILURIAN ROCKS OF GIRVAN. 545 



are absent. Immediately behind the keel is a narrow space free from radiating ribs. 

 Posteriorly, the radiating ribs become swollen and closer from behind forwards ; but 

 the ribs on the depressed cordate portion are stronger than those immediately in front 

 of them. 



Dimensions. — Fig. 30, PI. V., measures: antero-posteriorly, 17 mm.; dorso- 

 ventrally, 12 mm. ; from side to side, 11 mm. 



Locality. — Craighead and Balclatchie (Llandeilo). 



Observations. — Salter founded his species on a specimen from Craighead, and he 

 called it P. dipterus, var. rhomboidalis. Why the varietal name was added I cannot 

 see, and for the present I drop it. 



The shell is interesting because it already has all the characters and the peculiar 

 structure which I found in species of the genus in Carboniferous rocks. Indeed, it is 

 closely allied to C. alatum, de Koninck. For my reasons for orienting the shell so 

 that the elongate tube is posterior, and the internal structure and anatomy of the shell, 

 I would refer to my Monograph of British Carboniferous Lamellibranchiata, vol. i. 

 pp. 450-458. Developmental changes in the genus seem to have resulted chiefly in 

 giant forms and the growth of a fringe in C. hibernicum, the most specialised species of 

 the group. The general character of ornament seems to have persisted from Ordovician 

 to Carboniferous time. 



Cucullella antiqua, Sow. (PL IV., fig. 16.) 

 Cucull&a antiqua, Sow., Sil. Syst., pi. iii. figs, lb, 12a. 



I find one example of this species in the Gray Collection. The genus very closely 

 resembles Clidophorus, but differs from it in having a multidenticulate hinge plate of 

 the nucula type. Casts always show the deep slit posterior to the anterior adductor 

 muscle, which character is common to both genera. 



EXPLANATION OF PLATES. 



Plate I. 

 Fig. 1. Protopecten crenulatus, probably the right valve. Page 485. 

 Fig. 2. ,, vimineus, showing the external marking. Page 484. 



Fig. 3. ,, ,, showing the cast of the interior. Page 484. 



Fig. 4. Pterinea subfalcata, the left valve. Page 493. 

 Fig. 5. ,, „ a left valve. Page 493. 



Fig. 6. ,, elegans, a left valve, from a mould of the external cast. Page 494. 

 Fig. 7. ,, reticulata, a left valve, internal cast. Page 494. 

 Fig. 8. ,, „ a left valve, from a cast of the exterior. Page 494. 



Fig. 9. Pteronit.es ellipticus, the right valve with Fig. 9a, the impression of the hinge-plate. Page 496. 

 Fig. 10. Posidonomy a antiqua, a left valve. Page 492. 

 Fig. 11. Euthydesma alata, a left valve. Page 501. 



