ORIGIN AND SIGNIFICANCE OF SPINES 23 



Phylogeny of Spinous Forms. 



To interpret phylogeny in terms of ontogeny, according to 

 the law of morphogenesis, or recapitulation, is perhaps easier 

 than to trace a genetic sequence through a series of forms 

 having a considerable geologic range. Taking the ontogenies 

 of the animals already noticed, there is for the Pelecypoda the 

 prodissoconch, which is correlated by Jackson ^^ with Nucula, 

 and a Lower Silm-ian nuculoid radicle is assumed for the 

 Aviculidse and allied forms. The first dissoconch growth pro- 

 duces a shell resembling Rhombopteria, a Lower and Upper 

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

 in the phylogeny of Avicula (figure 25), Anomia, Spondylus, 

 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 

 genera, and are believed to indicate phylogenetic relationships. 

 It 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 "^ 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, ostrseiform, 

 squamous, concentric, and spinous growth becomes more 

 manifest, and at present most of the species show a great 

 development and differentiation of the spines. 



Thus, while Spondylus 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. 



