Hyatt.] 376 [June 7, 



•with Steph. Braikenridgii in those varieties which grow rapidly In size 

 and have narrow umbilici. In others the subcoronatum form is more 

 plainly discernible. This is precisely similar to the relationship 

 which exists between Brongniartii and Gervilii. 



A review of the general relations of the different series exhibits 

 some peculiarities worthy of our attention. If we start from Steph. 

 nodosum and compare the different species of each genetic twig or 

 branch, we are struck with the very distinct characteristics of each 

 series of forms. 



Steph. Bayleanum is decidedly retrogressive, the size and the invo- 

 lution of the whorls is less than in the type of nodosum. Humphrieai- 

 anum, on the other hand, acquires in succcessive forms finer ribs, 

 rounds the whorls of the adult and increases the amount of the in- 

 volution, and, in the highest forms, the elevation of the abdomen. 

 Steph. sidworonatum holds more closely to the type of Steph. nodosum, 

 and forms the centre of affinities for all the remaining groups. 



The comparison of these three main groups also reveals the very 

 interesting fact that Bayleanum and Humphriesianum have no de- 

 scendants; only the last of the three mentioned, subcoronatum, ap- 

 pears to have been fruitful in this respect. Bayleanum, in the course 

 of its growth, contracts the whorls at an early stage, thus replacing 

 the Pettos-like form by a more flattened, discoidal whorl in the adult 

 stage. Humphriesianum, on the contrary, increases in the relative size 

 of the whorls for a considerable time, but sooner or later shows the 

 effects of the contraction of the mouth parts, which appears at first as 

 a transitory characteristic near the mouth of each newly formed liv- 

 ing chamber. This contracted part is completely absorbed in the 

 younger and adult stages, when growth is resumed after each season 

 of rest, but not in the old. Therefore after a period more or less 

 prolonged, according to the size and growth force of the specimen, 

 the shell begins to diminish in the size of the whorl and the involu- 

 tion to decrease. This eventually, becomes very marked, especially 

 when it is accompanied, as it must be in extremely old specimens, 

 by the loss of the spines and ribs. The Pettos-like form is retained 

 for a longer period in the young than in Bayleanum. 



Steph. subcoronatum, on the other hand, retains the Pettos-like form 

 much longer than the other two, shows hardly any signs of decrease 

 in the rate of increase in the size of the whorl by growth, and there- 

 fore presents in many specimens no very marked old age changes in 

 the shells. It is altogether more like the parental nodosum or Pettos 







