part 4] THE EVOLUTION OF LIASSIC GASTEOPODS. 303 



(1) Development of Ornament. 



From an ontogenetic study it is possible to distinguish two 

 clearly-marked groups of turriculate gastropods in Liassic rocks : 



(i) A group in which spirals always appear before axials in development, 

 (ii) A g-roup in which axials always appear before spirals. 



The first group includes the Procerithidse. In typical species 

 of JProcerithium, after one or more smooth whorls at the apex, a 

 nearly median spiral appears ; in the succeeding whorl a second 

 (and posterior) spiral is developed, to be followed in many cases by 

 the development of axial ribs, which forma network with the spirals. 



On the other hand, in the development of Zygopleiira, after a 

 few smooth apical whorls, the earliest ornament to develop consists 

 of axial ribs. In Katoslra spiral threads are added to these in 

 later development, and it may therefore happen in certain cases 

 that the adult of Katosira resembles in some degree the adult of 

 Grocer itliium\ but, owing to the difference in the order of develop- 

 ment of the axials and spirals, there is no difiiculty in determining 

 to which group such homoeomorphic forms belong. 



Fig. 3. — Turbo sp. Lower Lias (capricornus zone)^ the 

 Brickworks, Stoneliouse (Gloucestersliire) . 



a = Specimen X 3; 6 = Embryonic whorls from the same XlO. 



This difference in the order of development of axials and spirals 

 would naturally be of value in other families : for instance, in 

 Pleurotomaria similis spirals appear before the axials, while in 

 species of Turbo the axials appear before the spirals (fig. 3). 



Changes in the shape of the whorl are largely associated with 

 increase in number and prominence of the spirals, particularly in 

 Proceritliium. The smooth embryonic whorls are simple and 

 rounded, becoming somewhat angular with the development of the 

 first spiral. When a second spiral is added, the whorl becomes feebly 

 bicarinate, and appears somewhat flat; and, even after a third spiral 

 has been intercalated, there is no marked change in the shape of 

 the whorl, the sutures appearing deep and wide, since they are 

 bounded by prominent spirals. As further spirals ai'e added, how- 

 ever, in more advanced species, the relative importance of the two 

 primary spirals decreases, and the whorl becomes more rounded 

 again. These changes in the shape of the whorl are to be seen 

 in the development of many species of Frocerithium (see fig. 5 a, 

 p. 305). 



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