208 C. T. Trechmann and D. Woolacott — 



On ray inquiring further in this matter Mr. Bolton writes : 

 " With respect to the shell of A. Vinti, I think that in the ' spat' 

 stage there was little or no calcification. I would not be sure, hut 

 I rather fancy that the shell is little more than a top integument, 

 and may possibly be represented by the periostracum at a later 

 stage. I am assuming this from the Glochidia larva of the common 

 Swan Mussel. I have not seen the latter for some years, but 

 I think the valves are well domed and hooked at one end, so that 

 the larvae can hang on to the gills of the parent; afterwards 

 calcification takes place in regular concentric layers around the 

 umbonal point, and the hooked condition is left together with the 

 tegumentary-like appearance ; and the shell assumes more or less 

 the appearance of the adult stage." 



It was, therefore, a matter of interest to me to try and observe 

 whether on any of the Claxheugh specimens one could see the 

 "spat" form in the act of growing into the A. Icevis form or 

 whether the umbones of the larger shells showed any trace of the 

 wrinkled stage of the spat. I examined all the available specimens 

 with this point in view. Certainly there are no specimens of 

 Anthracomya on the slabs of smaller size than " Ancylus" Vinti, 

 which is a fact in favour of the latter being actually the young stage 

 of the Anthracomyas associated with them. 



In one or two instances there seemed to be evidence of the 

 wrinkled periostracum of the " spat" stage remaining on the umbo, 

 though in nearly all the specimens after the calcification of the 

 interior of the valves of the spat, the character of the shell seems to 

 change rapidly and the periostracum to disappear. (See PI. Y, 

 Pigs. 6 and 7.) 



Notwithstanding these difficulties I am quite inclined to agree 

 with Mr. Bolton's interesting suggestion that all the bivalvular 

 organisms in this bed belong to different stages of growth of one 

 and the same shell, commencing with " Ancylus" Vinti and ending 

 with Anthracomya Phillipsi. 



One slab shows clear evidence of an " Ancylus" Vinti with 

 incipient calcified growth-lines developing round the anterior, 

 posterior, and lower margins. 



I may here remark that the apex of " Ancylus " Vinti does not 

 correspond with the beak of the more adult form Anthracomya 

 minima. In the former, in well-preserved examples, the rounded and 

 rather indefinite apex never occupies the hinge-line but is situated 

 well below it, and anterior to the middle of the shell, or eccentric, 

 as Kirkby described it. 



Anthracomya Phillipsi (Williamson). (PI. V, Pigs. 5-7.) 



Bivalves having the appearance of this form occur in great numbers 

 more or less crushed in the two bands of clay-ironstone in the 

 Claxheugh section. 



My specimens vary in size from 6 mm. long and 3 mm. high to 

 32 mm. long and 14 mm. high, and certainly appear tome to represent 

 various stages of growth of one and the same species. 



No other bivalves have been found in these beds, consequently the 



