I io INVERTEBRATE PALAEONTOLOGY 



progress, attaining its prime after an interval longer or 

 shorter than that of its fellows ; but there is commonly 

 some measure of similarity in the quality of structural 

 evolution in any homogenetic scries. Such correspond- 

 ence has been indicated above in the case of the Tetra- 

 branchiate Cephalopods. The Nautiloids, in spite of the 

 extreme deliberation of their progress, pursued lines of 

 specialization which can be correlated with those precipi- 

 tately followed by the Ammonoids. The principle of 

 parallelism of development in homogenetic forms (ortho- 

 genesis) seems applicable to all grades of classification. 

 Cases are not wanting where this morphogenetic corre- 

 spondence is emphasized by its contemporaneous occur- 

 rence in divergent stocks. Among the Echinoidea several 

 such phenomena occur. The Holectypoid genus Pileus 

 and the Diademoid family of Diplopodiidae types ulti- 

 mately homogenetic, but strikingly dissimilar in structure 

 and probably in habitat both show the unusual feature 

 of biserial pore-arrangement adapically, and both arose in 

 the Upper Jurassic. Again, the curious form Lanieria, an 

 Holectypoid with many affinities to Holectypus, acquired 

 in Upper Cretaceous times a type of ambulacral structure 

 identical with that of the contemporaneous Conulus^ with 

 which it has few other features in common. This case 

 is made more striking by the occurrence in the Lower 

 Cretaceous of the Lanieriid genus Discholectypus, with 

 similar ambulacral plating. Discholectypus can hardly 

 be ancestral to Lanieria, but its development of Conulus- 

 like structure coincides in time with the origin of the 

 Pyrwa-stock, from which Conulus is clearly descended. 

 The Nucleolitoid genus Trematopygus, whose phyletic 

 connexion with the Holectypoida must be very remote, 

 produced almost exactly the same type of structure in 

 Lower Cretaceous times. 

 The second acceleration of evolution occurs when a 



