ARIETITES RARICOSTATUS. 299 



out clearly the boundary line between the Lower and Middle Lias, and has a very 

 limited life in time. 



It presents two distinct forms, both of which are met with in the British Lias, and 

 each requires a separate description. The first variety (a) is the form which has been 

 figured by Zieten as the type of the species. It has straight prominent ribs, set far 

 apart, with wide concave spaces between. We count about twenty costse in the last 

 whorl of a shell of 60 millimetres in diameter; when the Ammonite is larger the whorls 

 have more prominent ribs, which become thicker at the margin. The fossil, PI. VII, 

 fig. 2, is a good example of this form ; in it, however, the ribs are more numerous, on 

 account of the size of the shell. Those which I have collected near Cheltenham, and 

 those found at Ballintoy, in the North of Ireland, belong to this variety. 



The second variety (b) has more numerous ribs, nearly thirty in the last whorl, in a 

 shell of 40 millimetres in diameter. The whorls are in general less depressed, and the 

 inner are covered with very delicate, close-set, regular ribs. The evolution of this 

 variety is very well shown in a mass of specimens I obtained from a bed of shale 

 near Charmouth. The young shells of 16 millimetres in diameter all have nume- 

 rous bent ribs, set very regularly apart, which cross the siphonal area without any 

 trace of the carina which appears in larger specimens ; so it may be said of this 

 Ammonite that the young shells have many more ribs in a whorl than the older 

 specimens. The carina first appears as a slender central sheath, which only assumes 

 the form of a rudimentary keel when the shell reaches from 20 to 25 millimetres in 

 diameter. This variety is collected in great numbers from the upper beds of the Lower 

 Lias Clay near Charmouth. The shells, however, are so highly charged with iron 

 sulphuret that they unfortunately soon fall to pieces in the drawers of the cabinet by the 

 decomposition of the mineral matrix. 



The septa are very distinct in this species, and removed rather wide apart. They 

 form three lobes, and three saddles composed of unequal parts. The siphonal lobe is a 

 little longer than the principal lateral lobe (PI. VII, fig. 5), and has numerous denticles on 

 its sides. The siphonal saddle is one third larger than the principal lateral lobe, and has 

 three groups of lobules nearly equal in size ; the principal lateral lobe is irregular in the 

 shape and disposition of its lateral denticles ; the lateral saddle is smaller than the 

 principal lateral lobe, and has two terminal lobules with small lateral branches ; the lower 

 lateral lobe is small, with several lateral denticles, and the auxiliary lobe ends in simple 

 digitations. 



The septa stand wide apart, and the lobe-line occupies the valley between every 

 second rib, as shown in fig. 6, drawn purposely to demonstrate this remarkable character 

 in the structure of A. raricostatus, which is really not only raricostate, but likewise 

 rariseptate, in all the specimens I have examined. 



The front view (PI. VII, fig. 4) shows the plain siphonal area deprived of the shell, 

 with the slender keel in the centre of the area; and the back view of the same shell shows 



