180 Eepoet of the State Geologist. 



aocording to the genera considered. The normal development of 

 a Orustacean is at present well known and has been determined for 

 very different groups ; the animal grows by the successive ap- 

 pea Induce of new segments in front of the last segment. But 

 among the Trilobites the three divisions of the body — hea<l, 

 thorax and pygidium — undergo differentiations more or less rap- 

 idly according to the cases, and often consist each of one piece. 

 Acceleration takes place in very various degrees. 



Acceleration of regression. — It is easy to understand that the 

 existence of such a phenomenon does not result in simplifying 

 the phenomena of Phylogeny already so complex. 



The theory of embryonic acceleration, when applied to organs 

 greatly modified, can be pushed too far in considering forms re- 

 mote from the ancestral type. And if the form under considera- 

 tion is a regressive form, such as we have seen examples of, it 

 may happen that the most recent animal has the semblance of 

 being the most ancient. "We will add, however, that in most 

 cases the acceleration of regression only occurs in a determinate 

 organ or system of organs ; the rest of the organism may enable 

 us to reconnect the links of the chain and to arrive at a knowl- 

 edge of the meaning of the evolution. 



An interesting example of the embarrassment resulting from 

 the existence of such phenomena is furnished by the group of 

 Chelonians. The osseous carapace which covers the body of the 

 Turtle is formed of different pieces ; some of these constitute the 

 independent dermal bones, not homologous, whilst others are 

 only flattened expansions of the ribs and spiny apophyses of the 

 vertebra, uniting one with another. The existence of this cara- 

 pace, which at the maximum of its development is continuous 

 and without interruption, constitutes the principal trait of spec- 

 ialization of the Chelonians, and distinguishes them from all 

 other Reptiles. It is natural to look for the primitive forms of 

 a group among those in which this process is most slightly indi- 

 cated. But in one group of Turtles, both living and fossil {Atheca)^ 

 the costal plates are but little developed and are far from being 

 united ; the dermal plates, which form the ventral plastron, are 

 also separated from each other, there lying between them empty 

 spaces or fontanelles. Among more specialized forms, the Trion- 

 yohidcB, the Chelydrida^ etc., we find the ossification of the cara- 

 pace more pronounced ; among the Emyda, the Testudinida, etc., 

 it is fully attained in the adult state. It is an interesting fact 

 that among the large fresh- water Turtles and others also, the 

 closing of the buckler takes place very late, the fontanelles re- 



