MORPHOLOGICAL DIFFERENTIATION. 333 



ing for deducing the organization of the animal; in that case the terms 

 internal side and external side are used, which prejudge nothing. A 

 vertical line running from the external to the internal side gives the 

 height; a second line, perpendicular to the preceding, gives the breadth, 

 or thickness of the turn. 



In the involute shells, the growth, as was first recognized by Reinecke, 

 and later verified by Leop. von Buch, takes place according to a definite 

 law. Moseley and Naumann show that the law of growth corresponds to 

 a logarithmic spiral; consequently, the height and breadth of all the turns 

 are in the same proportion; the quotient of the height of two successive 

 turns gives the rate of growth of the mouth in height; the quotient of 

 the corresponding breadths gives the rate of increase in breadth; the 

 quotient of the diameter of the entire shell by the height of the last turn 

 expresses the rate of growth of the discoid (Scheibenzunahme). The 

 calculations of Moseley and Naumann were afterwards confirmed by G. 

 Sandberger and Grabau. 



The constitution of the internal partitions (septa) which limit the differ- 

 ent air-chambers is of considerable importance. Their number varies 

 extraordinarily in the different genera and the different species, but it is 

 quite constant in one and the same species; they are at increasing inter- 

 vals from each other, according to law, proportionate to the growth of the 

 shell, and it is only the last two partitions (septa) which precede the final 

 chamber, which are at a somewhat less distance apart. Probably all the 

 chambers have successively served as dwelling-chambers, and it is only 

 after a new partition was formed that it was transformed into an air- 

 chamber, which was no longer in communication, except by the Siphon, 

 with the last chamber. The mud and the sand were not able, generally, to 

 penetrate into the interior of the fossil shells when they were buried in- 

 tact, except in the last chamber, or by the siphonal opening into the last 

 air-chamber only. This is the reason why the chambers are very often 

 not filled with rock, but are coated or filled with crystals of calcite, of 

 quartz, of pyrite, of celestite, of barite, etc., which have been precipi- 

 tated from the infiltrated chemical solutions. 



The line of attachment of the partition to the internal wall of the shell 

 is called the Suture. (See Fig. 27, p. 106, and Figs. 112-118, p. 346.) It 

 is not exteriorly visible unless the shell is removed or dissolved. It 

 is seen more distinctly on the fossil moulds, in which the shell is 

 wanting. In the Nautilus, and in many of the shells of fossil Tetra- 

 branchiates, the septa attach themselves to the internal surface of 

 the shell by a slightly arched sutural line. Moreover, very often the 

 line of the suture, on account of the undulating curvature and a flut- 

 ing of the septum, acquires a high degree of complication resembling 

 the branching of moss. There are all degrees of variation from lines 

 the most simple to those most complex. Besides, as the lines have essen- 

 tially the same sinuosity for all the specimens of one species, and on the 

 contrary show differences quite striking in different species and separate 

 genera, they furnish thus one of the most important systematic char- 

 acters. In the Nautilidae the lines of the sutures are generally simple 

 (Fig. 106); in the Goniatites and Clymenias (Fig. 112) the undulating and 

 slashed suture forms prominent saddles before and curved sinuses behind, 

 called lobes. A later differentiation is met with in the Cerat'tes, etc., 



