1916.] J. HORNELL: Indian Varieties and Races of Turbinella. IIg 
forms (Pl. XI, fig. rz). In fully adult specimens the angularity of the shoulder be- 
comes considerably reduced and obscured and the difference between this form and 
T. acuta is thereby much lessened (Pl. XI, fig. 10). The shell spiral is more lax than 
in any but a few of the most elongated Indian forms of T. pirum acuta; its apical 
angle varies between 77° and 82° in the specimens I have examined. The second or 
anterior row of periostracal nodes is also strongly developed, though less than those 
of the first. Whether these two rows of periostracal nodes are equivalent to the first 
and second or to the second and third in T. pirum obtusa and T. pirum acuta I can- 
not decide. In both the latter the second row is usually the only one conspicuously 
developed, but in the majority of T. pirum obtusa and a considerable proportion of 
T. pirum acuta, the first row is either quite vestigial or actually suppressed ; again in 
some specimens of T. pirum acuta and T. pirum globosa, the first row is quite 
strongly developed, only a little less prominent than the second. Besides these two 
prominent shoulder rows, there are three to four body ridges, low, moderately marked 
and continuous, not showing any sign of nodulation. The Andaman shell is pure white 
in colour and rather light in weight. The anterior canal is markedly elongated. In 
both shells examined the columellar plicae appear as three high, compressed ridges, 
with a fourth, anterior to the others, obscure and low as is usually the case in this 
species generally. In the fully mature individual the three large plicae are thick and 
truncate at the free edge; in the smaller, which has a thin and immature lip, thin 
and knife-edged. 
Very young specimens of this variety show the shoulder angle much less emphatic 
than in the adult, and thus approach closely to the acuta form. 
The suture between the last two whorls is characteristically deep, and this is a 
disadvantage commercially, as it renders bangles cut from these shells easily broken, 
the deep suture forming an emphatic plane of weakness. 
CONCLUSIONS. 
Consideration and comparison of the foregoing and other facts lead me to the 
conclusion that the immediate common ancestor of all these varieties was a shell of 
the acuta form, having a moderate apical index and well developed periostracal sculp- 
ture arranged in spiral lines of discontinuous prominences or nodes. The Rameswaram 
and Kathiawar race seem to me the nearest to this form though both these probably 
have the periostracal sculpture less well-marked than in the ancestralform. Varieties 
fusus, globosa and comorinensis are all closely related to this ancient acuta form, 
whereas the typical obtusa is the most divergent and most changed form. Var. fusus 
has undoubtedly been long isolated from the rest of the species and under this isola- 
tion has developed special features which, being continued, have now become fixed. It 
is therefore a moot point as to whether it is not entitled to specific rank. It certainly 
is on the line that divides variety from species, and is an excellent instance of a local 
assemblage of individuals evolving differences—causes we much perforce ignore— 
which, becoming permanent, are in course of producing a new species. 
