252 FAMILY NUMMULINIDA. 



lyi 



tubercles, composed of a variety of shcU-substance which reflects light much more strongly 

 than the rest, that the more glistening aspect of this type is chiefly due ; and I have not met in 

 it with any instances of that general brownisli coloration of the surface which is the ordinary 

 characteristic of the other. The most constant and remarkable distinctive feature of this type, 

 however, is the presence of a large hemispherical cluster of semi-transparent tubercles in the 

 centre of the spire. — Having had no difiiculty in setting apart a large number of specimens 

 agreeing very closely with each other, and differing from the rest, in all the foregoing cha- 

 racters, I should have arrived at the unhesitating conclusion that this type deserves to take 

 rank as a species of Operculina distinct from the preceding, were it not for the circumstance 

 that every here and tliere I met with an example in which the di0"erential characters were less 

 strongly marked than usual. Thus in some individuals which preserve the general propor- 

 tions of the spire, the green coloration is wanting, the large central cluster of tubercles is re- 

 placed by a single tubercle of extraordinary size, the septal bands of the neighbourhood are 

 not more tuberculated than in many examples of the ordinary type, and the rows of tubercles 

 over the chambers are either wanting altogether, or are not more prominent than in many 

 individuals of the preceding type. This evidence of the negative value to be attached 

 to the number or prominence of the tubercles as a specific character, is confirmed by a 

 curious fact of an opposite nature ; namely, that in certain individuals we find them deve- 

 loped to an extraordinary and obviously abnormal degree. — From the difliculty of deciding to 

 which type particular specimens are to be referred, I had been almost led to adopt the con- 

 clusion of the specific identity of this with the ordinary form ; when Dr. Gould, of Boston 

 (U. S.), kindly placed in my hands some Opcrcullnce which had been collected on the coast of 

 Japan by the recent American expedition to that country. These specimens combined in so 

 remarkable a manner the most distinctive features of the two types, — namely, the general 

 form and proportions of the one, with the umbilical hemispheric cluster of tubercles and the 

 general abundance of tubercular elevations characteristic of the other, — as to remove all doubt 

 from my mind with regard to their specific identity. Similar variations present themselves 

 among the dwarfed examples of this type which are found on the shores of temperate seas 

 ■ (^450), and also among the fossil forms of it which abound in the Eocene and other Tertiary 

 deposits. Tlie minute form most common in northern seas has been described and figured 

 by Prof. Williamson (ex,) under the designation oi Nonionina elegans ; but I fully agree with 

 Messrs. Parker and Rupert Jones (lxxx h, lxxxi), in identifying this as a varietal form of 

 Operculina complanata. 



439. Internal Structure. — The study of the internal organization of Operculina may be 

 prosecuted in two modes; — b}' the examination, under sufficient magnifying powers, of thin 

 transparent sections taken in difi'erent directions ; — and by viewing under a low power, as 

 opaque objects, fragments obtained by breaking the shell, especially (as Mr. Carter was the 

 first to suggest) after these have been allowed to absorb carmine or indigo by being placed 

 upon water in which either of these colours has been rubbed up. By the former method 

 alone can certain minutise of structure be detected ; but the latter is extremely serviceable in 

 enabling the observer to trace out the relations of various parts, which sections exhibit to him 

 disconnected from one another. Owing to the circumstance that large specimens of Opercutiua 

 are rarely if ever quite flat, it is next to impossible to make a section through the median 



