30 FOSSIL MOLLUSCA OF THE CHALK. 



elevated and tuberculated on the sides near the large and deep umbilicus; tubercles 

 10 to 12 on each whorl, large and ill-defined; inner whorls half concealed: aperture 

 transverse, nearly semicircular. 



Diameter, 10 inches; width of aperture, 5 inches. 



Very rare in the Grey Chalk of Wiltshire and Sussex. 



This rare Ammonite, of which only two specimens have been met with, is the only 

 species yet known in the Chalk of the Family of the Coronarii, so abundant in the middle 

 Oolites. 



A misprint in the explanation of Dr. Mantell's Plate xxii, where fig. 5, is called A. catinus 

 instead of A. navicularis, has led to this species being confounded with A. Mantellii, both 

 in the ' Paleontologie Fran^aise,' and in Bronn's * Index Paleontologicus.' 



P^cl^v <itscv5 

 14. Ammonites Portlocki, Sharpe. Plate XIII, fig. 2 and 3. 



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



A. testa injlatd, costatd, tuberculatd : ambulacris rotundatis : juniore, tubercnlis latera- 

 libus 10, costis 20 transversis, dorso interruptis ; adultd, tuberculis binis, dorsalibiis, 

 majoribus ; costis rarioribus : umbilico parvo, prof undo : aperturd transversim ovali : 

 sepforum marginibus lateraliter trilobatis, lobis trifidis. 



Shell gibbous, with few rounded whorls wider than high, and a small deep, smooth 

 umbiHcus : when one and a half to two inches in diameter the whorl has ten large blunt 

 tubercles on the middle of each side, and twenty strong rounded ribs, rising either singly 

 or in pairs from the tubercles, and curving slightly forward to near the middle of the back, 

 where they become obsolete ; when three inches in diameter only one rib rises at each 

 tubercle, which bears a second larger tubercle near the middle of the back, and the 

 intervening ribs disappear : in the internal cast the back has a shght medial groove : inner 

 whorls nearly half concealed : aperture transversely oval : septa with three trifid lateral 

 lobes. 



Diameter, 3 inches ; width of aperture 2 inches. 



Found in the hard Chalk of Tamlaght, in the county of Derry, by the Ordnance 

 Geological Surveyors. 



The young shell bears some resemblance to A. Jissicostatus, to which this species was 

 first referred, but it is distinguished by more transverse whorls and fewer ribs, and at a 

 later stage of growth by the large dorsal tubercles. I have named it after Lieutenant 

 Colonel Portlock, R.E., who conducted the Geological branch of the Ordnance Survey of 

 Ireland at the time the specimens were found, and first published a notice of them. 



