56 PEOF. n. H. swi:n^neiitox and me. a. e. tel'emax [vol. Ixxiil, 



Dr. O. Abel ^ considers that, while iiiany ammonites were adapted 

 for a swimming hfe, jet many of the ' one-sided ' forms that we 

 have mentioned had a benthic habit. He also sug-g^ested that the 

 presence of a keel in a nectonic animal would assist rapid motion. 

 As we liave seen, asymmetric siphuncles rarely occur among these 

 keeled forms, which presumably liad I'arely a benthic habit. 



Briefly then, although a displaced siphuncle and an 

 as^nnmetric suture may be most usually developed in 

 certain connected species, the existence of such ab- 

 normalities in a single specimen is not sound evidence 

 of genetic affinity. 



In conclusion, we wish to ex})ress our thanks to those who have 

 helped us : to W. D. Lang, M.A., F.G.S., for gifts of material ; 

 to Prof. J. W. Carr, M.A., F.G.S., for permission to examine the 

 collection in the Nottingham Museum ; and to Prof. W. S. Boulton, 

 F.Gr.S., for allowing us similar access to the collection in the 

 B i r min gh am Un i versify . 



EXPLANATION OF PLATES II-IV. 

 Plate II. 



Fig. 1. Apparatus used in survejang and levelling the surfaces of septa and 

 of whorls. 



2. Dactijliocerns commit ne (J, Sowerby), from Whitb3\ Specimen used 



for working out the development of the septum, the suture-line, and 

 for making septal sections. Diamet£r=7-.5 cm. X I- 



3. Portion of the whorl of another specimen of the same species filed 



away in steps, in order to show septal sections of successive septa. 

 X H. 



4. Series of septal sections drawn from this specimen. 



Plate III. 



Figs. 1-5. Development of the septum of Daetyliocevan commune. Figs. 2-5 



are frcmi the specimen s]K)\\Ti,in PI. II. fig. 2. 

 Fig. 1. Adult septum. X 4-5. (Text-fig. 4.) 



2. Septum at 2^ whorls. X 22. 



3. Do. at 2^ whorls. X 25. 



4. Do. at 1^ whorls. X 35. {The numbering of figs. 4 



5. Second septum. X 40. (Text-fig. 6, p. 36.) j & 5 should be reversed. 

 0. Almost adult septiim of TragnphyUnceras Josromhi (.1. Sowerby), from 



Lyme Regis. Diameter = 17 mm. X <>. , 



7. Adult septum of Sphceroceras hfoiuiiriarti (J. Sowerby). from Sher- 

 bf)rne. Diameter = 20 mm. X 2h. 



Plate IV. 



Figs. 1-3. Peritii^hinctes tripliratus, frtmi Market Rasen. 

 Fig. 1. Front view of an abnormal septum. Natural size. 



2. Ventral view of a portion of an outer whorl. Natural size. 



3. Last suture shown by this specimen. 



* ' Verbreitung & Lebensweise der Ammoniten " Verhandl. k.-k. Zool.-Botan. 

 Gesellsch. Wien, 1912. p. 83. 



