16 ACCELERATION OP DEVELOPMENT IN FOSSIL CEPHALOPODA 



mained in the Tirolites form, and septa, and developed little beyond it in 

 sculpture, but has inherited the traehyceran furrow. It is then a lower 

 form than the subgenus Traskites, but lower in the sense of being more 

 retrograde, that is more thoroughly retarded. Both are partial rever- 

 sions towards Tirolites by loss of characters, but both have retained the 

 furrow, which Tirolites never had, and which they have inherited from 

 the progressive ancestor Trachyceras. The genetic series is: Tirolites, 

 Lower Triassic ; Trachyceras, Middle and Upper Triassic ; and Clionites, 

 Upper Triassic. It should be stated here that Trachyceras is a polyphy- 

 letic genus, not all of its species coming from the line of Tirolites, but 

 some from the stock of Meekoceras; and it is not yet known to which 

 branch the type of the genus, Trachyceras Aon, belongs. 



Another case of arrest of development is seen in Paraganides, of the 

 Upper Triassic, PI. VI, figs. 22-26, where a member of theNannites group 

 has lagged behind its fellows until it is scarcely beyond Aganidcs of the 

 Devonian and Lower Carboniferous, but shows its inheritance from more 

 complex intermediate genera in the internal lobes. This is the last mem- 

 ber of a genetic series that began in Aganides, (PI. I, fig. 15), continued 

 in the fixed type Nannites (PI. Ill, figs. 4-8), and finally perished in the 

 retarded and reversionary Paraganides. 



Eeversion. 



When a form develops normally and then strikes back to its ances- 

 tral type we have real reversion. It is not known positively that we have 

 any examples of this, but the development of Lituites, of the Silurian, 

 PI. XIV, fig. 6, and of Baculites, of the Cretaceous, PI. XIII, figs. 1-9, is 

 probably to be explained in this way. The ancestral stock was Orthoceras, 

 PI. XIV, fig. 1 ; then came Cyrtoceras, PI. XIV, figs. 2 and 3 ; then Gyro- 

 ceras, PI. XIV, fig. 4; then coiled nautilian shells, PI. XIV, fig. 5, and 

 finally Lituites, after becoming coiled, strikes back at maturity to the 

 straight orthoceran type. Most degenerate types are reversionary, at 

 least in some characters, though none are probably completely so. 



Baculites among the ammonoids has a similar history. Its remote 

 ancestor was Orthoceras; then came Bactrites of the middle Paleozoic; 

 then the coiled Goniatites; then the Ammonite stock of Lytoceras; and 

 finally, after being coiled normally, it strikes back to the straight form 

 of its progenitor. The reversion is only partial in either case. Such 

 a partial reversion is seen also in Crioceras, of the Cretaceous, PI. XIV, 

 fig. 11, where the shell becomes uncoiled, and reverts partly to the 



