214 John Young — Shell Structure of Eiclmaldia. 



III. — Notes on the Shell Structure of 'Eichwaldia Capewelli, 



By John Young, F.G.S. 

 Hunterian Museum, University, Glasgow. 

 T) ECENTLY, while reading Dr. Davidson's description of his 

 V\i Eichioaldia Capewdli, Brit. Sil. Brach. Snp. pp. 140-1, T was 

 much interested with that paragraph in which he states, " It had 

 been suggested to me that the appearance of the sculpture of the 

 surface of E. Capewelli is precisely that of certain Polyzoa one is 

 accustomed to meet with in Palaeozoic rocks, a fact that had struck 

 Mr. E. Etheridge, jun., as well as Prof. Nicholson ; but to accept 

 the ornamentation as that of an encrusting Polyzoan we must sup- 

 pose that the shell had been entirely replaced by the parasite, as 

 sometimes occurs with Hydractinia. Sections, however, show a 

 perfectly homogeneous structure of the whole thickness of the shell." 



Remembering that we possessed a single example of this shell in 

 our Museum, a ventral valve, I determined on having a close ex- 

 amination of its surface, to see whether it would reveal any points of 

 structure other than that already noted. Dr. Davidson, Mon. Brit. 

 Sil. Brach. pp. 193-4, describes the ornamentation of the shell as 

 follows : — " Surface of both valves closely covered with raised, 

 thread-like ridges, forming all over the shell a network of more or 

 less regular six-sided cells, the bottom of the cells being iiat, and 

 margined by slightly raised hexagonal ridges. The sculptured sur- 

 face is very peculiar, and nearly resembles that seen on Disciva 

 (Trematis) punctata. The cells (not punctures or perforations) vary 

 much in size and shape, are small at the umbone and on the beak, 

 but gradually become larger as they near the middle of the shell, 

 becoming again smaller as they approach the frontal and lateral 

 margins. When there has been an interruption in the growth of 

 the shell, and which is indicated by a sharp concentric line, the cells 

 often begin again by being smaller. They also assume a greater or 

 lesser degree of regularity in their shape, some being almost tri- 

 angular, lozenge-shaped, or more or less regularly five or six-sided." 



An examination of our Hunterian specimen showed the ornamen- 

 tation of the surface to be as above described ; but on a portion of 

 the valve, where the raised hexagonal ridges that bound the cells had 

 been worn down through weathering of the surface, I noticed that 

 these ridges covered over and concealed, on other parts of the shell, 

 a numerous group of polygonal cells, that are at least one-fourth 

 the size of the regular hexagonal cells seen at the surface. On 

 communicating this fact to Dr. Davidson, he expressed interest in 

 what I had found, and very kindly sent me all his specirnens of 

 JE. Capewelli (British and American) to examine, and has also 

 allowed me to etch with acid and to polish certain of the specimens, 

 so as to prove whether this small polygonal shell structure existed 

 in any other of the specimens in which it is not visible on the sur- 

 face of the shell. I was glad to find that several of Dr. Davidson's 

 specimens confirmed what I had already found, and in a more clear 



