
924 THE AMERICAN NATURALIST. [VoL. XXXVI. 
the ovicapsule, the deposition of limy matter begins as a slender 
cone or elevated point along the line of the axis of the proto- 
conch, and as the larva grows the posterior part of the mantle 
secretes a shelly dome. Being thus cut off from the horny 
protoconch, the latter falls into shreds and is lost. The nucleus 
of the larva, still in the ovicapsule, then presents a slightly 
irregular dome, with a slender point rising from the apical 
To this apical point Dall has applied the term “ calca- 
In this case the protoconch proper was horny and decid- 
uous, while the first formed septum was calcareous and hence 
came to take the place of the protoconch when this was lost. Prob- 
ably the types of nuclear whorls named by Dall “ Caricella type "' 
and ** Scaphella type " from characteristic genera are secondary 
septal deposits of this type, as already suggested by that author. 
Characteristics of the Conch.— The simplest type of whorl of 
the gastropod conch (torteconch) is round and smooth, showing 
only the lines of growth. This type, which is virtually only a 
continuation of the primitive protoconch whorls, is seen in 
Natica and similar types, and in these the umbilication is gen- 
erally retained. Omitting Pleurotomaria ! 
and its allies, this type of gastropod is 
most characteristic of the Paleozoic strata, 
the chief modification being the elonga- 
tion of the spire, thus producing a lox- 
onemoid type. | 
One of the earliest modifications of the 
smooth shell is the appearance of spirally 
revolving, more or less elevated lines and 
xvii inet the formation of transverse elevated folds 
whorls x 10, showing bulbous OF ribs? (Fig. 7). Our observations are 
Tea weenie * still insufficient to settle the question as 
to the order of appearance of these two 
types of ornamentation. Where both are present the ribs in 
most cases are the first to appear to the unaided eye, the spirals 


1 The development of Pleurotomariidz and Bellerophontidz, and of Euompha- 
lus and other related genera, has been well described by Koken (Mewes Jahrb. für 
Mineralogie, Beilage, Bd. vi, pp. 305-483). 
2 These ribs must be distinguished from varices, such as occur in Scalaria, 
 Harpa, and Murex. 
