| 
ORIGIN OF DIFFERENTIALS. 49 
explanation of the observed phenomena; but that the characters, as in Weiss- 
mann’s theory, singly or in part could have affected the ovum when they first 
appeared, does not seem to be sustained by the facts. The examples cited above 
of the transmission of the characteristics of the asiphonula and cecosiphonula 
to the typembryo of the Ammonoidea give similar evidence with regard to the 
origin of embryonic characters, and are directly against Weissmann’s position. 
Barrande and Munier-Chalmas have tried unsuccessfully to prove the abso- 
lute invariability of differentials among fossil Cephalopoda by means of the great 
apparent differences between the embryos of Nautiloids and Ammonoids, but 
the discovery of a protoconch in Orthoceratidz has demonstrated their error,* 
and we confidently anticipate the discovery of some form in which the proto- 
conch will exhibit intermediate characters. 
The rostrum in the Ammonoids contrasts decidedly with the central sinus of 
the same region of the aperture in Nautiloidea, and is a differential of importance, 
which ought to be mentioned here. The rostrum of the mesozoic forms indicates 
that the Ammonoids did not possess the ambulatory pipe or hyponome? of Nau- 
tiloids, which causes the ventral sinus in the aperture and striae of growth in that 
order. As noted above, they could not have been swimmers in the same sense as 
the existing Nautilus, and they must have been for the most part strictly littoral 
crawlers. The habit of crawling as a slower mode of progression combined with 
varied habitats of shallower waters, may have been the cause of the higher spe- 
cialization and greater variety of forms and structures which they exhibit. The 
change from a ventral sinus to a rostrum in the aperture began among the higher 
Goniatites during paleozoic time, and is shown by the narrower ventral sinus 
of the aperture, and in the lines of growth on the shell. This sinus also is more 
distinctly marked, and is present oftener, in the devonian forms of Goniatitine 
than in the carboniferous species. 
The Clymening of the Devonian have very small ventral sinuses in many 
forms, and in others the hyponome may have been absent. The specimens ob- 
served by the author have not as a rule exhibited the lines of growth with great 
clearness, but many of Giimbel’s figures give the striations, and in some species 
they pass straight across the venter. The Ammonoids of the Trias are appar- 
ently completely transformed, the rostrum and the ventral saddle in the lines 
of growth indicating the constant absence of a hyponome. 
As the ambulatory hyponome disappeared, all the sutures became more 
complicated or ammonoidal, and, in correlation with the greater lengthening 
of ventral and dorsal lobes, the central zone of the septum changed from con- 
cave to convex. When one tries to attribute the origin of convexity in the 
septa, or the multiplication and lengthening of lobes, or the marginal digita- 
tions of the sutures among ammonoids to fortuitous variations, he finds at once 
that their history in the groups of the order is correlative with the phenomena 
of morphological equivalence. As was long ago observed by Von Buch, and 
1 Science, IIT., No. 52, 1884, p. 126; and also above pp. 10, 11. 
2 See Foss, Ceph. Mus. Comp. Zodl., Proc. Am. Ass. Adv. Science, XXXII, 1883, p. 340; also Science, 
III., No 52, 1884, p. 123; and above, p. 29. 
7 
