450 THE MORPHOLOGY OP 



thickness and in retaining the zigzag-style of ribbing a much longer 

 time in proportion to inclusion. It may well be designated joseudo- 

 procerum. 



In considering the further developments of the zigzag-series there 

 can be little doubt that both arhustigerus and planula ^ as figured 

 by d'Orbigny ^ are certainly further developments of that series, 

 probably derivations of crassizigzag a. The coarse ribs of their 

 inner whorls suggest the ^z^^za^-ornamentation very forcibly. It 

 seems also very possible that both Herveyi and macrocephalus 

 (d'Orbigny, Pal. frang., Terr, jurass. pis. 150, 151) may take their 

 rise from a form like pseudoprocerum. If this idea be correct it 

 may perhaps be advantageous to include the whole of the zigzag- 

 series in the genus Macrocephalites, Sutner.^ Two other species, 

 which may possibly be derived from the zigzag-series, are suhcon- 

 tractus, Morris and Lycett, and Morrisi, Oppel. 



Series y. 



This series is only known to me from Quenstedt's plates. The 

 example depicted in ' Ammoniten des Schwabischen Jura,' pi. 74, 

 fig. 5, is certainly quicker-coiled than zigzag or crassizigzag ; and 

 not improbably the name euryodus may be retained for it. Quen- 

 stedt's figures allow us to surmise that this form is the ancestor of 

 anceps, and in that case we have a remarkable instance of homo- 

 plastic development ; for in the anceps-series there is the same 

 ventral furrow as in ParJcinsonia, which was itself an earlier 

 development from the Stephanoceras-series. JS'ow anceps is the 

 type of a genus BeinecJceia ; and it seems very probable that the Peri- 

 sphinctes tyrannus and oxyptychus of Neumayr"^ may be developments 

 of forms of anceps. 



The accompanying table shows the genealogy of the species dealt 

 with. The zigzag-series is a branch from the coronate-radical mani- 

 festing the spinous stage at a later date than the Bumphriesianum- 

 series, to which it may be considered morphologically equivalent. 

 Just as the Humpliriesianum-series evolved forms belonging to the 

 costate stage, so the zigzag-series gave rise to similar costate forms 

 {procerum^ pseudoprocerum, &c.) ; so that there is a parallelism in the 

 developments of these two series as well as between the developments 

 which take their rise from zigzag and crassizigzag, when considered 

 in relation to each other. 



^ The planula of Hehl (Zieten, ' Die Verstein. Wiirtt,' pi. vii. fig. 5) is a 

 different species, apparently derived from the Martinsii-gvonp, looking at the 

 ornaments of its inner whorls. The ornamentation of its outer whorls suggests 

 that it is the ancestor of Lamberti, Sutherlandce, Maria, and hence Goliathus. 



^ ' Pal. fraug., Terr, jurass.' pis. 143, 144. 



^ It would be very interesting if homoplasy had produced from the zigzag-^iock 

 a later development so much like Sphcsroceras, which is an earlier derivation 

 of the same radical stem as that from which zigzag came. 



* ' Ceph. der Macrocephalenschichten,' Jahrb. d. k. k. geol. Reichsanst. Bd. xx. 

 (1870) pis. viii., ix. 



