Geology and Geography. 381 



ness of their course, the effects produced on the adjacent rocks, 

 and above all their regular parallelism. The author has exa- 

 mined eleven of these dykes within thirteen English miles mea- 

 sured across their course, which is nearly east and west, varying 

 only 4° or 5° to the north of west. The length of one of these 

 dykes, viz. that extending from Rinvoe Point to the bed of the 

 river Bonnet near Uromahair, county Leitrim, is sixty-three 

 miles, and its breadth is not more than forty feet. Another 

 runs from Broad Haven to Agrishead, a distance of forty-five 

 English miles ; and there are several others which have been 

 traced for forty miles. 



Mr Griffith spoke of a paper on a similar subject by Lieutenant 

 James, about to appear in the Transactions of the Geological 

 Society of Dublin. 



Captain Portlock mentioned that the direction of the dykes in 

 Tyrone is due north and south. 



4. Professor Phillips communicated the results of his investi- 

 gations on fossil Astacidae. He remarked of the species which 

 composed the genus Astacus, as at present existing, some are 

 found in salt and others in fresh water. There are two empiri- 

 cal characters by which the marine may be distinguished from 

 the fluviatile astaci ; in the marine species, the lateral divisions 

 of the tail are transversely divided, while in the fresh water 

 species the division is longitudinal. The marine species have 

 also large didactylous claws to the first pair of feet. All the 

 fossil species possess those characters which belong to the marine 

 division. Proceeding to investigate the question as to the pos- 

 sibility of identifying strata by means of their organic remains, 

 it was remarked that the study of the fossil species of the present 

 genus does not afford results very favourable to such a hypothesis. 

 Confining our attention to the oohte and lias, it was observed 

 that one species of astacus was found in every bed, from the 

 lowest of the lias to the uppermost of the oolite. One species 

 was confined to the coral rag ; four species were peculiar to the 

 grcensand ; some of the species were more local, and others ap- 

 pear to have had a wider geographical distribution, as is the case 

 with the astacus of the present day. 



Mr Greenough, Professor Sedgwick, and Mr Murchison, 

 complimented Professor Phillips highly on his successful labours, 



VOL. XIX. XO. XXXVIII. OCTOBER 1835. cc 



