TYPE AMMONITES— VII Oct. 



9. Vol. iii, PI. CCXII 



ALLIGATICERAS ALLIGATUM, Bean-Leckenby sp. 

 Vertumniceratan, vertumnus 



For the name Alligaticeras see T.A. iv, 1923, 57, and for Dichotomo- 

 sphinctes see PL DCL, T.A. vi, 1926. That Ammonites ingens will be 

 finally housed in Dichotomosphinctes is none too certain. 



The object of these corrections of plate-legends is to revise local 

 and chronological details, in accordance with later experience, of species 

 of Dichotomoceras itself and of others which have been attributed to it, 

 in order to make the following remarks understandable. 



Systematic 



Dichotomoceras : There are now three species — D. dichotomum, 

 (PI. CXXXIX), D. strumatum, (PI. DCCXLVII), and D. variocostatum, 

 Buckland sp., re-figured and described by Maud Healey, (Jur. Amm. ; 

 Q.J.G.S., lx, 1904, p. 58, pi. xi and Pal. Univ. 56, 1904). 



The order now given would appear to be the biological order of 

 the three forms. D. dichotomum reaches a diameter of 173 mm. without 

 showing any sign of the old-age character, large ribs, or even of ribs 

 set more distantly — a preliminary to the stage of big ribs. D. strumatum 

 shows wider spacing and enlargement of ribs at about 160 mm. diameter, 

 a huge-rib stage from about 190 to 230 mm. and a stage of distant, but 

 not so gross ribs afterwards : D. variocostatum passes over to the distant- 

 ribbed stage at about 180 mm. diam. ; but the subsequent enlarged ribs 

 are of low elevation comparable with the ribs of D. strumatum, as if the 

 gross-ribbed stage had been altogether skipped and there had been direct 

 passage to the less swollen rib-stage of D. strumatum. 



Thicknesses come next for consideration. D. dichotomum has a 

 thickness of about 30 per cent, of the diameter, with a tendency to 

 tail off as it grows larger. D. strumatum reaches 37 per cent, and then 

 shows decline. D. variocostatum yields to my measurements over ribs 

 about 33 per cent. — Miss Healey's measures of 29 per cent, are not 

 taken over the ribs, I surmise. These thickness-measurements give, 

 as I read them, a rise to individual maxima of thickness and decline 

 in old age, and a rise to a serial maximum declining in the same way 

 in the old age of the race, so that the last species D. variocostatum by 

 commencing decline earlier does not attain to the peak of D. strumatum. 



The suture-line shows similar development. In D. dichotomum it is 

 not so very strongly developed as in D. strumatum ; but in the old age 

 of D. strumatum there is conspicuous septal degeneration. Such 

 degeneration commences earlier in D. variocostatum, where, though the 

 suture-lines are not satisfactorily preserved, there seems to be no sign 

 of the elaborate longilobation of D. strumatum at its diameter of 

 139 mm. (PI. DCCXLVIId, figs. 1—3). 



The septal details seem to be the best evidence for placing, as the 

 terminal of the series, the species showing the greatest septal degenera- 

 tion, that is, D. variocostatum. However, it is doubtful if the geological 

 sequence accords with such presumed biological sequence. Certainly 

 the evidence is not much ; but it is worthy of attention. In a boring 

 at North Ferriby, East Yorkshire (T.A. iv, 39) D. dichotomum was 

 recognized at a depth of 30-35 feet, while at a depth of 50 feet " Dichoto- 

 moceras sp. stouter than D. dichotomum " occurred. This stouter form 



