4 
K. L. PRENDERGAST. 
inherent in the animal, while the degree of their development will depend 
upon external environmental conditions. Given the condition necessary for 
spine development, that is, a plentiful supply of CaCO.i, any species will 
probably develop spines in the same position in all its members. The inclina- 
tion of the spines to the body surface will depend upon the hardness of the 
sea-floor, arising at a high angle where the substratum is soft and being ad- 
herent where it is hard. In modern lamellibranchs the temperature of the 
water has a marked effect on the thickness of the shell, the Arctic forms 
having a thick shell and the warm water forms of the same species a shell 
thin almost to fragility. The difference between the Irwin forms, thin with 
short spines of small bore and those from Mt. Marmion, thick-shelled with 
heavy spines, of Tceniothcerus suhquadratus (Morris) is possibly due to the 
difference in temperature of the sea at the two localities. 
Both valves may be ornamented by ruga? and costff. The rugae are 
formed at the growing edge of the shell and are thus in inverse positions in 
the two valves. Costae are longitudinal folds perpendicular to the rugae. 
Where both are present, the surface is reticulated as seen in the Dictyoclostus 
group ; at the points of contact of the two series of folds an enlarged node 
may be produced, sometimes giving rise to a spine. Wrinkles are more in- 
definite than rugae, they may cross the visceral disc or be confined to the ears. 
The lamellae seen in Strophalosia and Aidosteges are quite distinct from both 
rugae and wrinkles. They are not folds but are the edges of the shell as it 
was laid down in successive layers. The distinctness of these layers one from 
another indicates that secretion of shell substance was not continuous but 
intermittent and seasonal, a period of secretion being followed by a resting 
phase. The control is a physical one but the response of the various genera 
is dependent on the sensitivity of the animal itself and is, therefore, a bio- 
logical factor. 
In the structure of the cardinal margin these shells show a wide diversity 
of form. In the Productus s.l. group a cardinal area is not ordinarily de- 
veloped. In Dictyoclostus callytharrensis n.sp. however, and in some Indian 
forms, e.g. Dictyoclostus indicus (Waagen) a narro-w concave “area” named 
by Schuchert and Cooper a ginglymus is often seen on the pedicle valve. It 
is to be noted that the ginglymus is only seen when the brachial valve has 
been displaced. This ginglymus is a groove along the cardinal margin of the 
pedicle valve into w’hich fits the narrow margin of the brachial valve hence 
greatly increasing the efficiency of the articulation of the valves. In the genus 
Produetorthis Schuchert and Cooper postulate that the ginglymus has arisen 
from the degeneration of the area. This explanation cannot hold for Dictyo- 
clostuSf as the early forms of the genus possess neither an area nor a gin- 
glymus. Whether the ginglymus can, conversely, develop into an area, present 
information does not allow us to state. It seems unlikely that development 
would take this course unless teeth are present to take over the function of 
the ginglymus as an aid to articulation. 
Both Aulosteges and Strophalosia possess a true area though its form is 
very different in the two genera. In Aulosteges it is linear in the earlier 
species, but rapidly develops among the later species to a high plane triangle 
produced ventrally and thus carrying the umbo of the pedicle valve away 
from the brachial valve. In Strophalosia the area is much smaller, remains 
linear throughout the genus and is produced over the hinge-line so that 
the umbo of the pedicle valve approaches the brachial valve. A narrower 
area is present in the brachial vah^e of Strophalosia. 
