﻿1851.] MURCHISON — SILURIAN ROCKS OF SCOTLAND. 175 



Were it not for the very large size of the cells, and the total ab- 

 sence of any diaphragm or covering to their wide mouths, it would 

 have been natural to refer these double-sided cellular plates to the 

 Bryozoa. I have to thank Prof. Milne-Edwards, however, for call- 

 ing my attention to their analogy with the Nidi of Gasteropod Mol- 

 luscs, the variety of arrangements among which are so striking*. If 

 they are of this nature, Murchisonia seems to be the only genus 

 likely to produce them, as that genus is* plentiful in both districts 

 where they occur. The name Nidulites might perhaps stand, whether 

 they belong to the Mollusca or otherwise. 



Locality. Mulloch Quarry, Dalquharran, in the shelly sandstones. 



Orricula? crassa, Hall. Pl. X. figs. 3, 4 ; and Woodcut, fig. 7. 

 0.1 crassa, Hall, Palseont. N.York, pl. 79. fig. 8. 



Round, generally wider than long, a little pointed anteriorly ; 

 umbo quite close to the anterior end, and much depressed ; fine close- 

 set striae radiate from it, and increase in number towards the mar- 

 gin ; they are crossed by still finer concentric lines (which show 

 strongest between the radiating striae), and by a few concentric rugae. 

 Shell thin. Breadth 8 lines. 



As this appears to be exactly the same with some smaller speci- 

 mens from the Utica slate of New York, I do not feel justified in 

 giving a new name ; but Hall describes his shell as marked by strong 

 concentric wrinkles and fine radiating striae, and his figure agrees 

 with this in making the latter far less conspicuous than the former : 

 in ours the reverse is the case, and the name crassa implies some thick- 

 ness, while ours is a very thin shell. The woodcut, p. 1 5 1, shows a very 

 well preserved specimen from Ardwell ; those in the plate, from Pen- 

 whapple Glen, show a laminated structure in the shells, and some 

 obscure traces of muscular impressions, which can, however, be hardly 

 distinguished from the irregular wrinkles produced by pressure. 



Locality. Ardwell and PenwhappleGlen, inOrthoceratite-flagstones. 



Pleurorhynchus dipterus, Salter, var. rhomboideus. 

 Pl. VIII. fig. 6. 



This is the third species of this genus as yet discovered in the Si- 

 lurian rocks of Britain. In America they are frequent. Valves trape- 

 zoidal, oblique, and (excluding both the produced ends) longer than 

 wide, convex, and with a broad keel running from the beak to the 

 front margin in a direction slightly oblique forwards. The keel 

 or central portion of each valve is a slightly elevated band, covered by 

 very regular and close-set striae of growth, crossed by a few obscure 

 longitudinal ribs ; it is most prominent (in this variety at least) on its 

 forward edge, which ends on the ventral margin in a projecting point. 

 The posterior wing is acutely triangular, extended backwards into a 



* Dr. Lund (Annates des Sc. Nat. 1834, 2nd ser. vol. i. pl. 6. f. 27, 28) has 

 figured some fossil egg-cases a good deal resembling the outer form ; but no natu- 

 ralist that I know of has seen a double series of equal-sized sacs arranged back to 

 back, after the manner of a honeycomb. 



