EXPLANATION OF THE PLATES. 3 
sharply marked with fine concentric strie. Mr. Davidson observes :— 
‘* Almost every specimen I have seen of this shell is more or less distorted, 
_ and subsequently but little can be said with certainty of its specific charac- 
ters :”’ from a specimen of the interior he had examined, he believed it to 
be so similar to those of Lingula proper, that he had provisionally placed 
it with that genus. 
Lower anv Upper Lineuta Frags of the Portmadoc district. 
Lower Tremapoc.—Wern, Borthwood; Trwyn-y-lago ; Cefn-Cyfarnedd, 
east of Pontnant-y-liadron, near Taihirion, on the Bala road, from Ffesti- 
niog, &c., where it is a common species. UppER TREMADOc.— Under Moel- 
y-gest, and Garth, opposite Portmadoc. 
HETEROPODA. 
Fic. 8.—a, b. BELLEROPHON ARFONENSIS, Salter. 
Memoirs Geological Survey, vol. iii., p. 349, pl. x., figs. 6, 7. 
a, From Memoirs’ Geological Survey, pl. x., fig. 6. View showing in- 
flated mouth : the form is, however, much altered by pressure. 
b. From the same plate, fig. 7. Back view. 
Urrer Tremapoc.—Moel-y-gest, Garth, Penrhyn, Penclogwyn, Tu-hwnt- 
yr-bwlch, Portmadoc. Passage beds, Lianerch. 
PTEROPODA. 
Fie. 9.—a, 6. THeca opercuLaTa, Salter. 
- Memoirs Geological Survey, vol. iii., p. 351, pl. x., figs. 22-24. 
a. From pl. x., fig. 22. View of flattened or dorsal side, showing opercu- 
lum. 
b. From fig. 22. With point restored. 
Lower Tremapoc.—Abundant in all the localities near Portmadoc, Borth-~ 
wood, Tyn-y-dre, Tyn-y-llan, North Wales. 
Fig. 10.—Conunaria Homrrayi, Salter. 
Memoirs Geological Survey, vol. iii., p. 354, pl. x., figs. 1-13. 
From Memoirs Geological Survey, pl. x., fig. 12. This large Pteropod shell 
is stated by Mr. Salter to occur four inches and a half long, and three 
quarters of an inch wide at the larger end, from which it gradually tapers 
to the point or apex ; the surface being ornamented with double ridges, eight 
in number. 
Urrer Tremapoc.—Garth Hill, abundant ; ‘luhwnt-yr-bwlch, Portmadoc, 
North Wales. 
