32 FOSSIL MOLLUSCA OF THE CHALK. 



tuberculated ribs, of which A. Rhotomagensis is usually taken as the type ; the septa have 

 not been well seen. 



17. Ammonites Oldhami, Sharpe. Plate XIV, fig. 2. 



Ammonites alteenatus, Portlock. Geology of Londonderry, &c., pp. 408 and 764. 



A. testa discoided : anfractibus ellipticis, transversim radiatis ; radiis alternatim 

 majoribus, dorso interruptis : dorso rotundato, impresso : umbilico magno : aperturd semi- 

 ovuli : lobis septorum trijidis ramosissimis. 



Shell discoidal, with slightly flattened, rounded whorls, ornamented with numerous 

 transverse ribs, alternately reaching the umbilicus, or stopping on the side of the whorl, but 

 all interrupted at the bac^k : umbilicus large and shallow, leaving one third of the inner 

 whorls visible : back marked in the internal cast with a slight medial groove : aperture 

 semi-elliptical : margins of the septa with three trifid, branching, lateral lobes. 



Diameter, 4 inches; height of last whorl. If inch, width of aperture \\ inch. 



Found in the hard Chalk of Dungiven, in the County of Derry, by the Ordnance 

 Geological Surveyors. 



This species is closely allied to A. Lewesiensis, Sow., having the same general form and 

 grooved back, and nearly similar septa ; the most obvious distinction is in the ribs, which 

 in A. Lewesiensis are equal, and all die out on the side of the whorl without reaching the 

 umbilicus. Having only seen one waterworn specimen of A. Oldhami, I offer the 

 description of it with some diffidence. It is named after Thomas Oldham, Esq., of the 

 Ordnance Geological Survey of Ireland. 



A. alternatus, Woodward, should be expunged from our catalogues, resting on an 

 execrably drawn figure of a broken specimen of A. interruptus ; which I learned from 

 seeing Woodward^s original drawing, in the possession of Mr. S. P. Woodward of the 

 British Museum. 



18. Ammonites leptonema, Sharpe. Plate XIV, fig. 3. 



A. testa discoided, compressiusculd, transversim siibtilissime costatd: costis numero- 

 sissimis, simplicibus, jlexuosis, ambulacrum totum amplectentibus : umbilico magno : dorso 

 angustato, rotundato : aperturd sagittato-ovatd. 



Shell discoidal, with fom- whorls rounded at the umbilicus, and slightly compressed 

 towards the back, enlarging rapidly, and ornamented by very numerous, equal, fine, thread- 

 like, flexuous ribs, which embrace the whole whorl ; umbilicus large and shallow ; back 

 narrow, and rounded ; aperture ovate. 



Diameter, 1 1 inch, height of last whorl, seven eighths ; width of aperture, five eighths 

 of an inch. 



Very rare in the Grey Chalk of Ventnor, Isle of Wight. 



