BLUE CHALK MARL. 95 



Tab. xix. fig. 26. A specimen covered with the shell, the anterior 

 side broken off. 



fig. 27. A cast of indurated marl, from Laughton Place. 



26. Inoceramus concentrkus. Tab. xix. fig. 19. {Geolog. Trans, vol. 

 V. tab. 1. fig. 4.) 



Subcordiform, longitudinally, concentrically sulcated; beaks converging, 

 recurved ; lower valve gibbous, produced ; margin entire. 



This shell was first described by Mr. Parkinson, in the Geological 

 Transactions : it had however been long known as a production of the 

 blue marl, but the imperfect state in which the specimens usually 

 occurred, prevented its characters from being previously ascertained. 

 It is a small species, of the curious genus formed by Mr. Sowerby, for the 

 reception of the large fibrous bivalve of the chalk. The specimens seldom 

 exceed 1 -2 inch in length. The shell is nearly cordiform, and marked by 

 gentle concentric grooves, the eminences between them being rounded. 

 The lower valve is gibbous, and produced at the beak nearly one-fifth of 

 its longest diameter ; the upper valve is smaller and more expanded. The 

 beaks are approximate, and shghtly recurved. 



It occurs in every known locality of the blue marl. 



Tab. xix. fig. 19, is a remarkably perfect specimen, still retaining a 

 considerable portion of the shell. 



fig. 15, represents the produced part of the beaks detaclied 



from the body of the shell ; examples of this kind are not unfrequent. 



27. Inoceramus sulcatus. Tab. xix. fig. 16. (Geolog. Trans, vol. v. 

 pi. 1. fig. 5.) 



Subcordiform longitudinally ; with deep, radiating, oblique, longitu- 

 dinal sulci ; beaks recurved, lower valve produced, margin undulated. 



In the position of the beaks, and general form of the valves, this 

 species corresponds with the former ; from which, however, it is remarkably 

 distinguished by its longitudinal furrows. These commence at the beak, 

 and radiate with an oblique curve towards the margin, enlarging as they 

 proceed. The ridges that separate the sulci are rounded, and are from 

 seven to nine on each valve. Mr. Parkinson observes, " that on the 



