108 V MEMORANDUM ON THE 



To begin, then, with the specimens before us, in the relative order of 

 their abundance, we have — ■ 



1. First, Numerous blocks of a compact greyish siiicious limestone, 

 (in some parts passing to sandstone), filled with shells and casts of a 

 small inequivalve, eared bivalve, which do not appear to differ from the 

 small Pecteus imbedded in the lias blocks from the Coast of Yorkshire, 

 which we owe to the liberality of Mr. Taylor. The shells themselves are 

 changed to a black colour as they are in that formation. As they are 

 mostly mutilated, it is not improbable that other genera may hereafter be 

 distinguished among them. One such I have recognized — ^a very transverse 

 bivalve, not unlike Unio, in external shape — but, as I could only find one 

 cast of it, I have not attempted to give it a name — with this genus of 

 Pecteus must be ranged two mutilated specimens, which we have in a dark 

 bluish black limestone, and which are only a variety— possibly only the 

 same shell in a more advanced stage of growth — ^The generic marks are 

 wanting, but by comparing them with a beautiful English specimen of 

 Mr. Taylor's, no doubt can remain as to their identity. 



2ndly. Many specimens of an inequivalved bivalve, which has been 

 changed into a white crystalline substance, and from its hardness pro- 

 bably contains much silex. They are imbedded in a hard slate of the 

 same bluish black colour, which is covered with small scales of mica— 

 They appear to belong to the genus Producta, and may be compared with 

 a specimen of the same genus, the Producta Scotica, in Mr. Calder's 

 collection, and a plate of the same in Ure's Geology. They differ some- 

 what from this species by the greater flatness of the lower valve ; but 

 as most of the specimens have suffered from compression, it is difficult to 

 ascertain what has been their natural shape. Besides the larger and 

 more abundant variety of which we have been speaking, there is a 

 smaller one, or rather some casts of one of its valves — the depression in 



