1142 



DR. E. GHOSH ON THE 



the shape, size, and the position of the anterior adductor muscle 

 may have something to do with the atrophy* of the extreme 

 anterior region of the body. The poor development of the byssus 

 apparatus and the absence of sense-organs are probably examples 

 of degeneration due to its fluviatile life. 



The a,nimal of Scaphula agrees more with the animals of 

 Byssarca Swainson, Barbatia Gray, and Acar Gray than those 

 of the other genera of the Arcidse in its great antero-posterior 

 elongation and in the inequality in size of the adductor muscles. 

 It further resembles the first two, especially in the widening of 

 the body from side to side, and in the great disparity of the 

 adductor muscles in size. But the foot and the visceral mass 

 are strongly affected in the present genus by the elongation of 

 the middle region of the body, whereas in Byssarca and Barbatia 

 these organs are comparatively less elongated, the elongation 



Text-figure 4, 



Heart (from the dorsal aspect) of S. celox. 



being mainly exhibited in the posterior retractor pedis muscles. 

 In Acar the body seems to be equally elongated in the middle 

 region (the above-mentioned parts being afiected) and in the 

 posterior region beyond the posterior adductor, the mantle only 

 being affected. The elongation of the posterior region of the 

 body is thus greatest in Acar, least in Byssarca, and attains an 

 intermediate condition in Scapliula. With regard to the heart 

 of Scaphida, we find an intermediate condition of the ventricles 

 between Byssarca (with a wide separation of the ventricles) 

 and Barbatia (in which they are only slightly separated), but 

 marked difference from Acar, which has a single, but distinctly, 

 bilobed ventricle. The position of the heart in relation to the 

 rectum might have been derived from a condition seen in the 

 above genera. Lastly, the projecting portion of the rectum seems 



* This is not referred to elsewhere. — Ed. 



