REPOET ON THE EADIOLARIA. 603 



Whilst in all tbe foregoing five families of Larcoidea the shell-form is regular and 

 their geometrical fundamental form is a lentellipsis (or a triaxial ellipsoid, with three 

 unequal isopolar dimensive axes), in the four remaining families of this suborder the shell 

 becomes bilateral or irregular (with the poles of the axes unequal). In two of these 

 families (Lithelida and Streblemida) the growth of the shell becomes spiral, in the last two 

 families (Soreumida and Phorticida) quite irregular. But as in all four families we 

 encounter the t}q3ieal trizonal medullary shell (or Larnacilla-shell), we are convinced that 

 they must be derived (wholly or partially) from the Larnacida. 



The Lithelida (the sixth family) are Larcoidea with spiral growth and bilateral 

 form (like Nautilus) ; therefore the spiral line lies in one plane and this spiral plane 

 divides the whole shell into two symmetrical halves (right and left). The axis of the 

 spiral (around which the shell winds) is a straight line, one of the three dimensive axes. In 

 the greater part of Lithelida (in the Larcospirida) the primordial or central chamber of the 

 polythalamous shell is a trizonal medullary shell or LaruaciUa-sheW, and the growth of 

 the first spiral turning begins as the development of the first (transverse) cortical 

 girdle of Ami^hipyle ; but as one wing (or lateral half) of this girdle grows more rapidly 

 than the other, it overgrows the latter and begins the spiral winding ; if the other wing 

 follow and overgrow the first, the spiral becomes double. Each of the three dimensive 

 girdles (of the Pylonida) may begin the spiral winding. There can be no doubt that 

 aU these Lithelida (the Larcospirida) must be derived from the Pylonida, by unequal 

 growth of the two halves of one girdle. Perhaps from those may also be derived the other 

 part of this family, the Spiremida {Spirema and LithcUus) ; in these the primordial 

 chamber of the spiral shell is simple, and may be derived by reduction of the original 

 Larnacilla-s\xe\\. But it is also possible that the Spiremida proceed directly from the 

 Larcarida, and that their ancestors did not possess a Larnacilla-s\\G\l. 



The Streblemida (the seventh family) are Larcoidea with spiral growth and 

 asymmetrical form of the polythalamous shell (like Helix or Turrilites); therefore the 

 spiral Line is twisted like a winding stair, and the spiral face is curved and divides the 

 shell into two unequal halves. The Streblemida have the same likeness and relation to 

 the turbinoid Foraminifera {Rotalia, Glohigerina, &c.) as the Lithelida to the nautiloid 

 Foraminifera {Poli/stomella, Nummulina, &c..). As in these calcareous Rhizopods also the 

 peculiar growth of the siliceous Streblemida begins from a primordial chamber to which 

 a variable number of roundish chambers (of increasing size) is apposed. But the building 

 of these chambers and of their septa is by no means so regvilar and complete as in the 

 greater number of turbinoid Foraminifera. As in a part of this family the primordial 

 chamber is a Larnacilla-shell, these also may be derived from the Larnacida, but the 

 other part (with simple central chamber) is perhaps produced directly from the Larcarida. 



The eighth family, Soreumida, is perhaps derived from the Streblemida by loss of the 

 spiral growth. The polythalamous shell is similar to the latter, but the chambers arc 



