EEPORT ON THE RADIOLARIA. 689 



ascending (like Helix). Therefore in these latter the geometrical fundamental form of 

 the shell is asymmetrical or "dysdipleural," whereas in the Lithelida bilateral-symmetrical 

 or " eudipleural." The lentelliptical or nearly spherical shell may be divided by a median 

 section into two symmetrical halves ; the right half is the mirror image of the left half. 



When in 1862 I founded the family Lithelida in my Monograph (p. 515), I knew 

 only one genus, Lithelius, with two species. The rich material of the Challenger 

 collection contains a great number of similar spirally constructed Larcoidea, so that 

 at the present time we may distinguish at least six genera. These belong to two 

 different subfamilies, which may possibly be afterwards better separated as families. 

 The first subfamily, Spiremida, possess a simple, spherical or subspherical, medullary shell ; 

 the second subfamily, Larcospirida, possess a trizonal or Xar??ac?7?a-shaped medullary shell. 

 No doubt these latter must be derived from Pylonida, as we ol^serve all stages of 

 development starting from a simple Trizonium; but perhaps also the Spiremida have the 

 same origin, their simple, spherical or subspherical, medullary shell being derived from a 

 trizonal or Larnacilla-sha'ped medullary shell by reduction. 



The general appearance in both subfamilies of the Lithelida is quite the same, and it 

 requires a careful study of the medullary shell to distinguish certainly the Spiremida from 

 the Larcospirida. This distinction is often not easy, particularly in the larger forms ; 

 the shell is often very opaque and difficult to understand. Only in one position, if the 

 spiral axis be parallel to the axis of the eye of the observer, and the spiral plane be therefore 

 fully seen in the optical plane of the microscope, the spiral line (or the axial section of the 

 latticed spiral lamella) is distinctly observed; in all other positions the figure of the spiral 

 is more or less indistinct, and the whole microscopical image often quite intricate and 

 confused. The sufficient study of this family requires therefore the contemplation of the 

 shell from different sides, and is the more difiicult, as the variabiUty of the Lithelida — 

 as of the Pylonida — is extraordinarily great. 



The description which I gave of Lithelius (1862) in my Monograph is in some 

 points erroneous, and was afterwards (1879) corrected by R. Hertwig, who explained 

 particularly the near relation of it to Tetrapyle. Indeed the intermediate forms between 

 the Lithefida and the Pylonida are so numerous and so evident in all stages of 

 development, that the derivation of the former (at least of the Larcospirida) from the 

 latter is quite clear. The analogy between the structure of the Lithefida and the 

 calcareous (foraminiferous) Alveolinida is not so complete as I supposed it to he in mj' 

 Monograph (1862); particularly the formation of the small chambers between the 

 turnings of the spiral lamella is much more complete in the Alveofinida than in the 

 Lithefida. 



The cortical shell of all Lithelida has the same geometrical fundamental form as 

 Nautilus or as the nautiloid Polythalamia [Polystomella, Nummulites, &c.) ; therefore 

 the shell is dipleural, being divided by the median plane into two symmetrical lateral halves. 



(ZOOL. CHALL. EXP. PART XL. — 1885.) Rv 87 



