GENUS OPERCULINA. '_>:;7 



elongated spots of transparent substance (fig. 12, b, /A, around wliicli the tubuli arc crowded 

 together, as if they had been displaced by its interposition. Such spots occasionally present 

 themselves in the ordinary type of Opercitlhui ; but it is of course in sections of the strongly 

 tuberculated variety that we find them most marked, and this especially in the investing 

 layers of the central region, where, from the size and upproximation of the tubercles, the 

 spaces between them are rendered almost opafpie by the crowding together of the tubuli. The 

 tubuli are generally seen, moreover, to be somewhat more crowded in the neighbourhood of 

 the septal bands. The greatest local development of the non-tubular substance is seen in 

 those varieties which have a large elevated tubercle in the centre ; a section of such a variety 

 has been shown in Fig. XLII, d, where it is seen that this tubercle springs from the spiral 

 lamina of the innermost convolution, but that it increases in diameter as it is built-upon (so 

 to speak) by each of the whorls by which this is subsequently invested. 



444. The texture of the shell-substance which bounds the chambers at the outer margin 

 of each whorl (Plate XVII, fig. I, a, a), however, presents a marked departure from that of 

 the spiral lamina ; a difference analogous to that which was first indicated by me in iSnmmu- 

 I'lles (xii), and which has been found by MM. D'Archiac and Haime (i) to present itself so 

 constantly in that genus, that they give to this marginal portion the special designation 

 "bourrelet." The speciahty in the structure of this part of the shell of Opercnlina was 

 recognised by Mr. Carter (xvii), who described it as made up of an aggregation of fusiform 

 spicules, and hence designated it as " the spicular cord."* Now I am very familiar with the 

 appearance which has led him to take this view of its nature, since it is one which is commonly 

 presented by thin sections that pass either through the median plane or parallel to it, as shown 

 in Plate XVII, fig. 3, «'" a"; but this is only one out of many aspects which are presented 

 by sections taken in various directions ; and a comparison of the whole seems to me to 

 lead to quite a different conclusion, — namely, that the supposed spicular composition of this 

 " marginal cord " (as it may be appropriately termed) is due to the peculiar manner in which 

 the homogeneous substance of which it is composed is traversed by the set of canals that are 

 correctly described by Mr. Carter as forming the " marginal plexus." For if the section 

 should happen to traverse a portion of this plexus in which the canals form a tolerably regular 

 network of elongated meshes in one plane, the appearance delineated in fig. 3 is pre- 

 sented ; but just as often a less complete layer of the network is traversed by the section, 



* Mr. Carter !ias recently (xxiii a) criticised my account of the structure of the " marginal cord," 

 and has re-stated his reasons for persisting in his original account of its "spicular" structure. So fiir 

 from finding in tliis latter statemeut any reason for modifying my own views, I draw from it additional 

 reason for believing that Mr. Carter has been misled by the results of the metliod of examination on 

 which he seems to place most reliance ; since by " cutting off taugentially portions of tliis cord with a 

 small sharp scalpel," lie must have almost necessarily found his sections frittered into the semblance of 

 an aggregation of spicules. The true structure of the cord is much more certainly shown by sections 

 made in various directions, and carefully reduced by rubbing down wliilst attached to glass; of such 

 sections I have numbers in ray possession. It may be well for me to add that MM. D'Archiac and 

 Ilainie have been equally unable with myself to admit the correctness of Mr. Carter's interpretation of 

 the appearances he has witnessed. 



33 



