SONNINIA ACANTHODES. 419 



proportion to size less conspicuous, if anything. The development of the suture- 

 line is noticeable. In PI. LXXIV, fig. 6, it is shown that the suture-line of the 

 young shell has an asymmetrical L due to anisometry of the lateral lobules. In 

 the larger specimen — the suture-line taken three-quarters of a whorl later than in 

 the smaller one — the anisometry of the lateral lobules is still more marked. In 

 another suture-line from the same fossil, taken a whorl later, the anisometry of the 

 lateral lobules of L is very pronounced ; the inner lobule is very much abbreviated, 

 and may be considered as aborted. 



Sonn. spinifera was placed in connection with the acantlwdes- stock (p. 335), and 

 the anisometry of the lateral lobules of L is similar to what obtains in Sonn. 

 ptycta and Sonn. cymatera. The septation is, in fact, very like that of the latter, 

 but the external ornamentation is very unlike. There is no need now to remark 

 on the difference between the adults of these shells — they are obvious from 

 the figures in PI. C ; but the young shells are more alike, and some pains were 

 taken to show the differences (p. 336). 



One mistake was made — the ribs of the young spinifera were said to be " more 

 reclining " than those of cymatera. This is hardly correct. The ribs of the young 

 spinifera are somewhat variable in slope, but tend on the whole to become rather 

 more upright with age ; those of cymatera become rather more reclinate. It is in 

 the earlier whorls of the young shell that spinifera shows more reclinate ribs than 

 cymatera ; later on the balance is rather the other way. The adult spinifera now 

 figured shows practically upright ribs on the last whorl, and is in marked contrast 

 to the adult cymatera. 



(3. Costas strong. 



Sonninia acanthodes, S. Buclcman. Plate C, fig. 8. 



See Plates LVIII, LIX, LX, and Plate LXIII, fig. 1, and page 

 319. 



Add to description : suture-line with long lobes, L with intra-axial, anisosce- 

 loid, inequicellate terminal lobule ; lateral lobules somewhat anisometric, but both 

 fairly developed. 



Portions of the outlines of two septa of the type are shown in PI. C, fig. 8. 



