64 G.V. MATTHEW: ILLUSTRATIONS OF 



tral face ; aud the side slopes of the operculum exhibit a few faint, distant, radiating striae. 



ie/ig-^/i, (exclusive of the larval tubule), 8 to 10 mm; tubule, IJ to 2 mm. Width, as 

 preserved in the shales, 2 to 2| mm. 



Horizon aud Locality. In the fine dark grey shales of Div. l.rf, at Porter's Brook, St. 

 Martin's. 



Diplot"heca Acadica, Hartt, sp. (Plate VI. Figs. 6 and 6a.) 



HyoUthes Antdicus, Hartt, sp. U.S. G-eol. Surv., Bull. 10. 



llieca Acadica, Hartt, mss. See the same Bulletin. 



Mr.Walcott's description (reversing the terms "dorsal" and " ventral") is as follows : — 



" Form, an elongate triangular pyramid, tapering gradually and uniformly to an acute 

 extremity. Transverse section subtriangular, about twice as wide as high ; the lateral 

 angles acute from compression in the specimens in the collection. Dorsal face slightly 

 arched ; anterior margin extending forward in a semi-circular subspatulate extension. 

 Ventral surface rather strongly convex. Aperture unknown, but undoubtedly oblique, 

 judging from the character of the extension of the dorsal side. — Operculum unknown. 

 — Surface of the shell marked by concentric lines of growth parallel to the margin of the 

 aperture, and exceedingly fine longitudinal strife, visible only by the aid of a strong 

 magnifier. — In general form this species approaches A''ery closely to HyoUthes Americauus, 

 Bill. (Can. Nat. N. S., 18*72. Vol. VI. p. 215), but equally so the Devonian H adis, Hall, 

 (Pal. N. Y., Vol. V. Part 2, p. 19*7) except in the more rounded ventral side. — Owing to the 

 imperfect condition of preservation of the species illustrated from the Menevian group of 

 Wales, it is difficult to make comparisons with them. Professor Hartt's specific name is 

 retained, as the probabilités are that the form is different from the American Potsdam and 

 Georgian species, although allied to H primwdialis, Hall, (16th Ann. Eep. State Cab. Nat. 

 Hist., page 135, 1863), and also the Menevian forms of the genus in Wales." 



Formation and Locality. Cambrian, St. John formation, Ratcliff 's Mill Stream, N.B. 



This species is the most abundant of the larger Hyolithoid shells occurring in the 

 lower i^art of the St. John group, and I may add the following to Mr. Walcott's descrip- 

 tion of the typical form : — 



The form of the operculum associated with this species seems to indicate that the 

 ventral side was regularly rounded and the dorsal somewhat flattened. The dorsal surface 

 is often impressed near the edge of the shell by the longitudinal furrow corresponding to 

 the margin of the phragmated sheath of the ventral side. One or two F-shaped seams also 

 mark the surface of the body cavity near the inner end, and probably indicate the edges 

 of the last two septa ; in some examples a narrow band with transverse strite, along the 

 axial line, simulates a siphon. 



An operculum which is common in the shales containing this species has the follow- 

 ing characters : nucleus about one sixth of the diameter from the ventral side ; depressed, 

 owing to the strong upward flexure of the ventral slope. The ventral slope has about six 

 radiating gi'ooves or striœ on each side ; the striœ do not quite reach the margin, and are 

 arched slightly toward the extremities of the ventral slope. The side slopes are marked 

 by about four straight radiating strite, between which other lighter stria? are sometimes 

 intercalated. The dorsal slope is large, flat, about as long as wide, aud marked near the 



