644 



THE LOWER SILURIAN PERIOD. 



Lingnla 

 Matthevd. 



Fig. 22L Lingula Matthewi, Hartt, MS. (Fig. 221). Dorsal valve, 

 • — circular in outline or very slightly wider than long, 

 extremely flat, the convexity being scarcely noticeable ; 

 shell very thin ; on each side a segment such as would 

 be cut off by a chord running from the umbo to the 

 extremity of the transverse diameter, is slightly turned up 

 on the margin. 



Inside, a strong mesial ridge, rounded and of moderate width, 

 runs from the umbo to a point a little beyond the middle of the shell ; 

 at the umbo this ridge bears a small nail-head-like process or swelling, 

 and there are two minute and extremely short secondary ridges, 

 originating from the head of the primary, and extending obliquely 

 backwards. Inner surface marked with numerous indistinct and 

 irregular concentric strige ; outer surface not visible. 



I have found one perfect dorsal valve in a piece of slate sent me 

 by Mr G. F. Matthew from Coldbrook. 



Lingula^ n. s., Hartt, MS. Differs from the above in being almost 

 straight in front, broadly rounded at the sides and naiTOwed towards 

 and pointed at the umbo. It was also larger, thicker, and more 

 convex. Ratcliffe's Millstream, Hartt. 



Obolella transversa, Hartt, MS. A very small, transversely oval 

 species, from Coldbrook, St John. 



Discina Acadica, Hartt, MS. (Fig. 222). Shell elliptical in out- 

 line ; sides more or less straight. Conical, but very depressed. Apex 

 Fig. 222. apparently central. Surface marked with a number of 

 deep concentric irregular sharp furrows, not always con- 

 tinuous, and often breaking up into smaller grooves ; and 

 all these seem at times to be impressed with lighter lines 

 running nearly parallel with them. Of the large furrows, 

 from nine to ten can usually be counted. The whole 

 surface of the shell is marked with a great number of delicate raised 

 lines radiating from the summit to the circumference, and just visible 

 to the naked eye. Rather rare in the Trilobite shale at Ratcliffe's 

 Mill. The shell appears to have been thin, and is probably much 

 compressed vertically. Collected by N. B. Survey and J. W. Hartt. 

 Orthis BilUngsi, Hartt, MS. (Fig. 223). Shell subquadrate to semi- 

 circular, broader than long; greatestwidth at the hinge-line; moderately 

 convex ; greatest thickness at about the middle, de- 

 pressed in front. Hinge-line straight. Dorsal valve 

 semi-circular or subquadrate, depressed, with a 

 shallow sinus running from the umbo to the front. 

 Orthis BilUngsi. Umbo not elevated above the hinge-area, which is 



Discina 

 Acadica. 



Fig. 223. 



