BIOLOGICAL PALAEONTOLOGY 97 



and verification, in palaeontological material. Although 

 full series of growth - stages of species can rarely be 

 collected, a large proportion of Invertebrates retain 

 the products of their early growth throughout life, so 

 that a single specimen can, by dissection or analysis, 

 be made to yield evidence of its ontogeny. Thus 

 Ammonites added successive whorls during life, but 

 did not destroy the first-formed parts of the shell. By 

 " unwrapping " an Ammonite (removing one chamber 

 at a time) the whole post-larval history of shell-growth 

 is gradually revealed in inverse order, until the proto- 

 conch is reached. Similarly, Gastropods display their 

 successive ontogenetic characters, although the exposed 

 position in which the early whorls are usually left (at 

 the apex of the spire) often results in damage or removal. 

 Pelecypods and Brachiopods show in the umbonal 

 regions products of early shell-formation, and may 

 occasionally retain their prodissoconchs or protegula. 

 The basal part of the theca of a coral has the first 

 simple arrangement of septa and dissepiments, while 

 more advanced stages of complication can be traced 

 upwards towards the calyx. 



Post-larval ontogeny is replete with difficulties like 

 those that complicate embryology. The short time 

 available in individual life for " revision " of age-long 

 evolutional history results in compression, omission, and 

 even disturbance of sequence, of stages passed through. 

 Developments of late introduction may be impressed 

 upon structures of primitive simplicity. Thus, while 

 the usual type of Gastropod protoconch is a smooth, 

 slightly coiled shell, features of ornament characteristic 

 of more advanced specialization may occur upon it, 

 as has been well shown by Grabau in the Fusidae. 

 But in spite of complexities, the principle of ontogenetic 

 stages gives generally satisfactory indication of phyletic 

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