308 



PROCEEDINGS OF THE GEOLOGICAL, SOCIETY. [May 19, 



We now come to the Lower Limburg or Lower Tongrian beds, 

 consisting of clayey greensand 20 feet thick, to which the 108 fossils 

 enumerated in the "Lower" column of Table IX. p. 312, belong, and 

 in which Ostrea ventilabrum is everywhere a common and conspicu- 

 ous fossil. The uppermost beds of this deposit, which I saw at 

 Lethen and at Grimittingen, where they are close to the green marls 

 (c of Table VIII. above mentioned), are characterized, observes 

 M. Bosquet, by the abundance of Turritella crenulata. 



Upper' portion of Lower Limburg, Table VIII. p. 304 {Lower Tongrian) . 

 Most abundant species of Fossil Shells according to M. Bosquet. 



Relative 

 abundance. 



Corbula pisum, 5'ow 1 



Lucina gracilis, A'ys^ 1 



Pectunculus lunulatus, iVys^ ... 1 

 Area sulcicosta, iVys^ 2 



Relative 

 abundance. 

 Ostrea ventilabrum, Goldf. ... 6 



cochlear, Poli 3 



Dentalium acutum, Heb 1 



Turritella crenulata, iVy^^ 3 



The lower beds of the same are seen at Hoesselt near Kleyn 

 Spawen and at Grimittingen. 



Most abundant species of Fossil Shells in the lower portion of the 

 fossiliferous Lower Limburg beds. 



Corbula pisum , Sow 



Crassatella intermedia, Nyst 

 Isocardia transversa, Nyst 



Cardita latisulca, iVys^ 



Area sulcicosta, Nyst 



Mytilus Nystii, Kickx 



Relative 

 abundance. 

 .. 1 

 .. 1 

 .. 2 

 .. 1 

 .. 1 

 3 



Relative 

 abundance 

 Janira (Pecten) incurvata, Bosq. 1 

 Ostrea ventilabrum, Goldf. ... 5 





Dentalium acutum, Hebert ... 1 

 Voluta suturalis, Nyst 1 



Pecten reconditus, Brander 



.. 2 



Professor Forbes refers these lower beds, which are purely marine, 

 to the upper part of his median (or coralline) zone. He supposes 

 that the inferior beds indicate a muddy bottom, which was at a 

 somewhat greater depth in the same zone than that in which the 

 superior ones were formed. 



It will be seen by consulting Table IX. p. 312, that the Lower 

 Limburg beds out of 106 species of mollusca have no less than 68 

 peculiar to themselves, only 38 species passing upwards into the 

 Middle and Upper Limburg divisions. All these 106 species are 

 purely marine, except Corbulomya complanata, found at Lethen in 

 the upper part of the bed. 



The distinctness of so many of these Lower Limburg fossils from 

 the species occurring in the beds immediately above, arises no doub t 

 in a great degree from a difference in stations, or in the fauna of the 

 the median as compared to that of the perilittoral zone. But the 

 changes in time may also have been great during those ages when the 

 sea and a river were contending for the occupation of the area in 

 which the middle Limburg strata were thrown down. 



