FIFTH, OK AGASSICERAN BRANCH. 203 



first mode of development for two and a quarter volutions. The increase is 

 gradual, so that well denned pilse are not produced until the last of the third or 

 first quarter of the fourth whorl. An English specimen of variety A has large 

 tubercles on the second quarter of the third whorl. These continue at long 

 intervals, only about two to a quarter of a whorl. The true pilaa do not begin 

 to appear until the third quarter of the fourth volution. Even then these are 

 exceedingly oblique and fold-like, and, though they soon assume the aspect of 

 true pilae, retain their obliquity until the second quarter of the fifth volution. 

 Another English specimen shows acceleration by developing similar tubercles 

 on the last quarter of the second volution, and also by the development of per- 

 fect straight pilae on the second quarter of the fourth volution. A third speci- 

 men of variety E, from Lyme Regis, has tubercles beginning on the last quarter 

 of the second whorl, and perfect pilae on the last part of the first quarter of the 

 fourth volution. 



Vak. Smithi. 



The British Museum possesses a card with several young specimens of the 

 variety Smithi, which at first appear to be identical with the full grown Icevigatum 

 or striaries. They have short living chambers, becoming more or less contracted 

 at the apertures, and fold-like pilae with striations. They have not, however, the 

 peculiar form of the young of Icevigatum, Plate VIII. Fig. 10, 13, nor similar 

 sutural digitations, and the living chamber is about three fourths of a volution in 

 length. It is interesting to note here the fact, that acceleration in the develop- 

 ment of the tubercles and pilae occurs in variety E, which has more involute 

 shells than other varieties. 



Ast. obtasum has characteristic sutures, and the number of specimens usually 

 exhibiting these parts were also favorable for testing the diagnostic value of the 

 proportions of the lobes and saddles in the definition of the species. We have 

 made large numbers of measurements, of which the following are selections. 

 The fractions express the differences between the abdominal and superior lateral 

 lobes, or the inferior and superior lateral saddles. The abdominal lobe and infe- 

 rior lateral saddles being always longer, the fraction expresses at the same time 

 the proportionate lengths in equal divisions of these two. 



A specimen of variety B, from Robin Hood's Bay, has three sutures on the 

 third and two on the fourth quarter of the fifth volution, with a difference of 

 only two fifths between lobes and saddles, and one septum on the third quar- 

 ter, in which the lobes are equal, the saddles remaining unchanged. One from 

 Teuchsloch bei Bempflingen has lobes varying from one fifth to one third, and 

 saddles from one sixth to two thirds. A specimen of variety C, from Boll, has 

 these proportions in none of its sutures, but either two fifths or one half between 

 the lobes, and one fifth or one third between the saddles. Comparisons were 

 made upon sutures of the same age in each case. Another specimen of this 

 variety from Boll, having similar gibbous sides, has similar proportions between 

 the lobes and saddles, with exception of the eighth suture, in which the difference 



