18 C. E. Beecher — Origin and Significance of /Spines. 



Aviculidfe and allied forms. The first dissoconch growth pro- 

 duces a shell resembling P/w7nbopteria, a Lower and Upper 

 Silurian type, and this is taken to represent the second stage 

 in the phylogen \uof Avicula (figure 25), Anomia, Spondylns, 

 etc. Continuing the development of Spondylus, it is found 

 by Jackson that it passes successively through stages which 

 may be correlated with Pterinopecten (Devonian), Avicu- 

 lopecten (Devonian), Pecten (Carboniferous ?), and Hinnites 

 (Trias), while finally it assumes true spondyliform characters. 

 These correlations agree with the geologic sequence of the 



fenera, and are believed to indicate phylogenetic relationships. 

 t may be further remarked that the early species of Spondyli 

 are more truly pecteniform and hinnitiform than the later 

 ones. The genus ranges from the Trias to the present. 

 Zittel 73 remarks that "the oldest species are small, thin-shelled, 

 and seldom much ornamented." Even in the Cretaceous, the 

 majority of species are not far removed from Pecten and 

 Hinnites. During the Tertiary, the irregular, ostrgeiform, 

 squamous, concentric, and spinous growth becomes more mani- 

 fest, and at present most of the species show a great develop- 

 ment and differentiation of the spines. 



Thus, while Sjpondylus is normally considered as a spinose 

 genus and the species are familiarly known as Spiny Oysters, 

 yet as it is traced back in geological history, the forms become 

 less and less spinose, and their affinities and appearances are 

 more and more in accord with non-spinose genera, until finally 

 the prototype is a smooth, simple, delicate, unornamented 

 shell. 



The simple antlers of the young Deer and Elk correspond 

 in type with those of the adults of the Middle Tertiary Deer 

 (Lydekker 44 ), and it may be therefore assumed that the great 

 number of branches and tines is a modern development. 

 Further back in the Tertiary, the ancestors of the Deer were 

 without antlers, thus representing in phylogeny the new-born 

 Deer of the living type. These correlations are made from 

 comparisons of chronogenesis, or development in time, and 

 ontogenesis, or development in the individual. 



An example of a different kind will now be given to show more 

 clearly a genetic sequence in forms. Among the Braehiopods, 

 Atrypa hystrix represents one of the terminal members or 

 species of aline of varietal and specific differentiation, extend- 

 ing through the Silurian and Devonian. The type commonly 

 known as Atrypa retic%daris appears to have had its inception 

 during the Ordovician; yet in the Silurian, ,it is found as a 

 conspicious and fully developed form. Here, also, it has quite 

 a wide range of variation, but there seems to be an insensible 



