GREY CHALK MARL. 113 



the form of the septa or dissepiments, are well preserved. It was disr 

 covered in a marl bank at Middleham, in the winter of 1814. 



The following are casts of the young shell. 



Tab. xxi. fig. 1. Nearly smooth, the spire oblique, and much com- 

 pressed. 



fig. 4. In this example a few linear sulci, and the markings 



of the septa, are the only indications of the species. 



fig. 8. This very elegant and perfect cast is from Middleham. 



It is deeply channelled by the broad transverse sulci, and is an excellent 

 example of the characters of the young shell. 



The small smooth Nautih so frequently met with in various localities 

 of the marl, are imperfect specimens of N. elegans in a young state. In a 

 few instances the septa are composed of sulphuret of iron, and the cham- 

 bers lined with crystals of carbonate of Hme. 



Localities. Hamsey, Stoneham, OfFham, Middleham, Kanscombe, 

 Firle, &c. 



28. Ammonites Mantelli (of Sowerby.) Tab. xxix. fig. 9. Tab. xxii. 



fig- 1- 



Discoidal, subumbilicate : volutions subrotund, costate, one-third in- 

 serted ; costae tubercular, transverse, alternately annular, with from two to 

 eight rows of tubercles ; ambit flattish, with two rows of marginal tubercles ; 

 septa very sinuous ; siphunculus external. 



The number and disposition of the ribs and tubercles of this species 

 are so various, that although it is one of the most abundant productions 

 of the grey marl, its specific characters are not easily defined. 



The general form of the shell is discoidal, the volutions (which when 

 perfect are nearly cylindrical,) being flattened by compression, as in the 

 specimens figured by Mr. Sowerby. The inner wreaths, in those which 

 are compressed, are nearly two-thirds concealed, but in more perfect 

 examples are less deeply inserted. The costae are round, and extend 

 alternately across the whorls, the intermediate ones embracing about two- 

 thirds of the volutions. The tubercles constitute the following varieties. 



Var. costata : with two rows of tubercles, tab. xxi, fig. 9. Two tuber- 



Q 



