52 OKHAMANDAL MARINE ZOOLOGY REPORT 



Kutch. Series of younger generations give almost practically the same ratio in one 

 case 114y\mm. to 102 mm., and in another 125^ mm. to 115 mm. We may, therefore, 

 take the ratio of length to lateral height or depth as 11 to 10, both for adults and 

 immature individuals. 



This slight inferiority in the height is due to peripheral growth being largely 

 arrested along the central portion of the dorsal line, where a linear hinge forms the 

 junction between the two valves. In consequence, adult shells show a straight edge 

 along the most dorsal region, measuring approximately one-fifth to one-fourth of the 

 extreme length of the valves. This particular feature, however, may exhibit consider- 

 able variation ; the hinge region in some shells may even assume an obtuse-angled 

 outline. 



The right valve is almost flat, the left slightly, but appreciably, convex externally. 

 In many adult individuals the mid-posterior region of both valves shows a single wide 

 shallow lateral depression or contortion, extending some distance inwards from 

 the margin, and with its convex aspect to the left side of the shell. There is 

 occasionally a similar but much less distinct bending in the mid-anterior region 

 of the shell, vis-d-vis to the posterior one. These two lateral contortions exhibit a 

 considerable degree of variation both in the extent or area in which they involve the 

 valves and in their relative proportions. Thus specimens are seen where the anterior 

 folding is larger than the posterior, and in others one or other may be practically 

 non-existent. 



Such contortion as is indicated here is developed to a maximum in Placuna sella, 

 a large and massive species of distinct saddle shape, separated from Placuna placenta 

 principally for this reason. In this connection it is interesting to see occasionally 

 among old individuals of the latter species, instances where contortion is sufficiently 

 well marked to exhibit suggestive approximation to weakly-contorted specimens of 

 Placuna sella. 



I have said above that the outline of the shell is sub-orbicular. So it is typically, 

 but rather because of the inevitable damage to delicate shell growth that takes place 

 from time to time as the shell increases in age, and which tends to remove more or 

 less completely any thin outgrowths that depart from the circular outline. 



In very young individuals of 2 cms. and thereabouts in diameter, the sub-orbicular 

 form without any marginal outgrowths is normal, and such show the straight dorsal 

 hinge line clearly, and a slight unbonar beak. In slightly older specimens, where 

 the shell has attained a diameter of about 5 cms., it is found that the portions of the 

 dorsal margin of the shell at each end of the hinge line begin to grow more rapidly 

 than the rest of the margin, and thus form one or several rounded ear or wing-like 

 lobes at either end of the hinge (Plate I., fig. 1). With the growth of the shell these 

 ears may become very prominent, and attain a length of from one to two centimetres 

 (Plate II., fig. 2, a.e.}. Frequently in young shells two, or even three, of these " lobes" 



