ROBSON : Olf SHELL MONSTKOSITIES. 



275 



more certain information concerning its origin than " from a pearl ? " 

 The lloytil College of Surgeons' specimen is "presumed" by the 

 catalogue to come from a species of Tridacna. They have been 

 examined by many competent authorities, who concur in referring 

 them to Tridacna, though only at liazard. Indeed, if they are to be 

 accepted as growths from a moUuscan sliell, there can hardly be any 

 refuge from this opinion. No one of the many autliorities who liave 

 seen the specimens has actually seen or lieard of precisely similar 

 gx'owths, and recourse has been had to the literature of shell growth 

 and teratology in vain. 



It is hoped that the supposition tliat tliey are referable to Tridacna 

 will be subsequently endorsed by some observer. But in any case 

 the ])resent note is rather a call for information than an assertion 

 of fact. 



The growths in (jueslion are irregular hollow masses of aragonite, 

 with a slight internal lining of a duhious organic nature. 



Fig. 2. Half natural size. 



The specimen from the College of Surgeons' Museum (Fig. 2) 

 measures 14 X 10 cm. and is rather heart-shaped. The obtuse end 

 has had a portion sawn off it. The smaller specimen, measuring 

 8-5x5'9cm., is flattish and roughlj' circular, and possesses a hollow 

 stalk or narrow neck of an irregular shape, which looks as if it served 

 to connect it to a surface of attachment. 



The nearest analogy that suggests itself to the author is the 

 occurrence of ' blisters ' in the pearl oyster Ifeleagrina margaritifera. 

 These 'blisters' are irregular excrescences of nacre on the inner 

 surface of the valve of the oyster, and are of course very much smaller 

 than these supposed Tridacna monstrosities. They are formed around 

 bodies that have intruded themselves between the mantle and the shell, 

 such as pearls developed in the mantle, and subsequently detached 

 therefrom, or foreign bodies such as sand-grains or intruding animals. 



