404 J. C. Ilansel-PJeydeU — Geology of Dorset. 



Ammonites margaritatus. A layer of marl, six feet thick, divides 

 these lower beds of the Middle Lias frora about seventy feet of sand, 

 ■which are in their turn succeeded by ninety feet of Upper Lias clay, 

 near the base of which is a remarkable conglomerate of pebbles, 

 seldom more than an inch in thickness, imbedded in a ferruginous 

 matrix with Oolitic granules. The clays beneath assume more the 

 appearance of the marlstone of other districts. Above this great clay- 

 bed come about 140 feet of brown micaceous sand containing layers 

 of sandstone. Until lately they were named the sands of the 

 Inferior Oolite, but are now generally assigned to the Upper Lias. 

 At Fourfoot Hill, to the east of Eype's Mouth, there is another 

 downthrow of nearly 500 feet, by which the upper beds of the 

 Inferior Oolite are brought below the beach-line. The Lias appears 

 again at Bridport Harbour ; also the sands, which attain a thickness 

 of 120 or 130 feet at Burton Bradstock Cliff; further eastward they 

 are covered by beds of Inferior Oolite and Fuller's Earth. 



These Midford Sands attain a thickness of 140 feet in the neigh- 

 bourhood of Sherborne, and are traversed by horizontal layers of 

 indurated sandstone nodules, which rarely contain organic remains. 

 It will be observed from the above account of the Dorsetshire Liassic 

 beds, that they are confined to the south-western corner of the county. 

 The Lias vies with the Oxford and Kimmeridge Clays in paleeonto- 

 logical treasures, especially the almost comparatively extinct Saurian 

 and Cephalopod families. With the exception of the Crocodile and 

 the Alligator, the present representatives of the Saurian family are 

 insignificant and harmless; and of the chambered Cephalopods, 

 which probably formed the chief food of their contemporary gigantic 

 reptiles, and had an extensive generic distribution during the Oolitic 

 period, Nautilus pompilius is the only existing representative. 



The reptiles form a prominent feature of the Liassic formation in a 

 palcBontological point of view, the variety and abundance of which 

 have been proved by the exertions of the late Miss Mary Anning 

 and of ]\tr. J. W. Marder ; of these the flying Saurians are the rarest; 

 Dr. Buckland was the first to give evidence of their presence in the 

 Lias. In 1838 Professor Owen obtained a skull with a few other 

 parts of the skeleton of the same or a closely allied species ; in 

 March, 1868, Mr. Henry Woodward, when visiting Lyme Eegis, 

 secured for the British Museum an entire tail with its bony tendons, 

 20^ inches in length, closely resembling the tail of BhampJiorhjncJius 

 from the Solenhofen stone of Bavaria. In August following the Earl 

 of Enniskillen sent up to Professor Owen a slab with the greater 

 part of tlie bones and head of Dimorpliodon, having a portion of the 

 caudal series of vertebrae identical with those forming the entire 

 detached tail already obtained ; ' thus establishing the existence of a 

 new generic form — Dimorpliodon, in addition to Rhamphorhynchus and 

 Pterodactylus proper. The most distinguishing features of the genus 

 are the association in the same mouth of fish-like teeth, with long 



1 See Geol. Mag., 1870, Vol. VII. p. 98. Dr. Buckland also observed some of 

 these curious vertebraj in his original specimen of JDhiiorphodon [Pterodactylus) 

 macromjx, but, strange to say, considered them to be cervical. 



