Oolitic Formations in the vicinity of Minchinhampton. 27 



beneath are entirely barren of organic relics, and gradually and insensibly 

 merge into the Upper Lias. 



The general diminutive appearance which the Great Oolite shells present 

 when compared with those of the other Oolrtic rocks cannot fail to be noticed. 

 In species which have a considerable vertical range this fact is rendered 

 particularly striking : thus, but for a perfect indentity of markings, Trigonia 

 coatata reduced to the size of a bean, and sometimes even to a pea, would 

 scarcely be regarded as the representative of the large Inferior Oolite shell : 

 higher in the Oolites it again attains its pristine dimensions. Astarte 

 excavata too, without the aid of a large series for comparison, would not be 

 recognised ; the shell becomes small, depressed, and the costae rendered 

 almost obsolete. Modiola plirata, which reappears in the upper beds of 

 Great Oolite, nearly loses its plicae, and acquires a compressed angular form. 

 The changes of size which Lucina lyrata undergoes is still more remarkable. 

 In the lower rag-stone it is of full dimensions ; in the Nerinea bed or 

 Fimbria bed it is reduced to one-fourth its former bulk ; in the upper rag- 

 stone it is again large ; in the shelly beds of the Great Oolite it is rare, but 

 is again reduced to the dimensions of the Fimbria bed ; lastly, in the upper 

 beds of the Great Oolite it is again abundant and of its standard bulk. 



Next as to the gregarious habits of certain species: — Bussage, a small 

 hamlet north of the vale of Brimscomb, produces in its shelly Great Oolite 

 a large assemblage ol an undescribed species of Terebratula somewhat 

 resembling T. globata, but very rarely having both valves in juxtaposition, 

 and seldom found in any other locality. In the limestone beds of the upper 

 fossiliferous series, one locality has produced a dense assemblage of a fine 

 bivalve which seems to belong to the new genus Cardilla of Deshayes, 

 although generally in these beds it is rare. The compact structure of the 

 rock renders it nearly impossible to disengage them in a perfect state, but 

 the fine striae of the shell are well preserved, and the character of the 

 species evident ; its sudden advance in size when compared with the small 

 fossil shell upon which the genus was founded is remarkable, and justifies 

 the specific appellation of grandis. 



The association of species at the locality in question is curious; the 

 whiteness of the Cardilla limestone displays every testaceous fragment in 

 strong relief, and enables us to discover that the Cardilla is the only bivalve, 

 and that it is accompanied by a Purpuroidea, and more sparingly by three 

 large Naticae, all of which probably constituted checks upon its super- 

 abundance. Monotis radiata occurs by myriads immediately beneath the 

 planking bed on Minchinhampton Common, and the gregarious habits of 

 Perna mytiloides may often be shown in a small hand specimen of rock. 

 Cardium Beaumonti, Archiac, is found only in the upper beds, where, in 

 abundance, they rival the Perna mytiloides, of the lower series. In spots 

 where the rock becomes a barren sandstone far away from all detritus of 

 shells, and probably deposited at greater depths, a cluster of Pholadomya 

 concentrica or P. Murchisoniea sometimes appears ; nor are any shells of 

 the genus Pholadomya ever found in the shelly beds of the Great Oolite ; 

 they are likewise absent in the Fimbria bed of the Inferior Oolite, and it 

 may be safely predicted, that they never will be found in the shelly roe- 

 stone of the vicinity of Cheltenham ; these beds were evidently deposited 

 in a shallow sea, and portions of them even possess a littoral character. 

 The little knowledge we possess of the habits of the recent Pholadomya 

 Candida is in exact accordance with this fact. At one locality the upper 

 beds have produced a dense colony of Terebratula media to the exclusion 

 of all other shells. Lucina lyrata, Pholadomya truncata, P. nana, 

 Ceromya excentrica, and Ceromya semistriata are likewise never found 

 isolated. „ 



The changes of external characters produced by growth alone form 

 another interesting subject for study, and have occasionally become a 

 source of error and confusion. Two examples will sufficiently illustrate 

 this. The large and elegant new species of Lima (L. varians) has a 



