Zool.— Vol. I.] BANCROFT— CHELYOSOMA PRODUCTUM. 329 



other hand, all of its distinguishing characteristics are of a 

 very exceptional kind. Thus, the division of the test into 

 plates is shared by only one other species of tunicate, 

 Forbessella tessellata, while the attachment of the muscles 

 and their definite relations with the plates are, so far as 

 known, unique. The lacunar character of the blood-vessels 

 in the test, asymmetrical hypophysial gland, and single peri- 

 pharyngeal band are also unusual. 



Of the two species of the genus, C. macleayanum presents 

 on the whole fewer aberrant characters and is nearer the 

 direct line of descent from some Corella-like ancestor. Its 

 exceedingly depressed form is more unusual than the shape 

 of the western species; but on the other hand, the youngest 

 individuals of C. froductum are very much flatter than the 

 older ones, thus indicating a depressed ancestor. The dif- 

 ference in the musculature also furnishes very strong 

 evidence for believing C. macleayanum the more primitive 

 type ; for we can easily see how a division of the test into 

 plates with the muscles extending perpendicularly to the 

 hinges might be developed from a form with an irregular 

 mass of muscles confined to the dorsal side of the animal. 

 Then, when the plates are once formed, the condition in 

 C. firodicctum could be obtained by increasing the periph- 

 eral muscles. On the other hand, it is very difficult to 

 see how the peripheral plates could ever have been devel- 

 oped directly by a system of muscles extending parallel to 

 the hinges. In the more recent development of C. fro- 

 ductum then, we have what appears to be the reason for its 

 unstable condition, and the attendant variability in those 

 characters which distinguish it from the other member of 

 the genus. 



