294 
SUPPT.EMENT TO 'IMIE BRITISH 
secondly, by etching the surface of the valve with weak acid. When the perforations 
happen to be filled np with a mineral substance ditFering from the lime of the shell, and 
when the valve is etched, the perforations stand out in fine relief on the surface of the shell 
like little spines. In one specimen the perforations stood out in little patches from the 
irregular tilling up of the pores, bnt in most examples they are pretty regular. In the etched 
valves, the perforations that are mostly seen standing out as short spines are those of the 
second series ; they project from the shell in a slanting manner, and with a little longer 
etching would be entirely rem.oved from the surface of the shell, leaving only the smaller 
sort visible. Both these processes, adds Mr. Young, gradually wear out the larger series 
of pores, leaving only the smallest series visible under the microscope, or by means of a 
strong pocket lens. The different kinds of pores and perforations will be seen in the 
magnified illustrations in Sup., Fl. XXXIV, figs. I6a, 17. 
Much credit is due to Mr. J. Young for his persevering researches not only into the 
shell-structure of this species, but likewise of so many others. Prof. King, who has 
also examined the specimens operated upon by Mr. Young, observes, in a letter to that 
gentleman, that it seems to him possible that the smaller series of pores are not those seen 
on the inside surface of the shell, and that it is the second series of pores that pass from 
one surface of the shell to the other; but Mr. Young contends that the pores of the 
second series are too wide apart on the outer surface of the shell to form the minute 
perforations seen on its inner surface, and I agree with him in the view he has expressed. 
In a well-preserved and carefully etched valve that Mr. Young has just sent me, the 
smaller series of pores are beautifully seen on its inner surface, arranged in lines corre- 
sponding to the ribs, and with the intervening spaces free of pores (Sup., PI. XXXIV, 
fig. 16a). This s[)ecimen clearly shows that only the smaller series of pores reach the 
inner surface of the shell. I have given enlarged illustrations, showing the three kinds 
of pores, in fig. 17, at a, d, and c. Pig. IQa shows the inner surface of the valve with 
the smaller perforations, c, which traverse the thickness of the valve. 
33. Orthis resupinata, Marfin. Dav., Garb. Mon. p. 130. PI. XXIX, figs. 1—4; 
PI. XXX, figs. 1—5. 
It is rare to find good weathered specimens of this species showing the perforations. 
Here we have the larger spine pores which do not perforate the shell and a very minute 
series of pores or tubuli which accordingly paiss through the entire thickness of the shell, 
but there is no second series of small spine pores as is seen in Orthis Miclielini. 
