104b YORKSHIRE TYPE AMMONITES— II June 



104. DEROCERAS IMPAVIDUM, nov. 

 (Plate CIV) 



Description 

 [M. Simpson, 1855, pp. 65, 66] 



" 93. A. spicatus, Simp. — Volutions 9, exposed, outer whorl more 

 than one-fifth the diameter, inner edge prominent, back rounded, sides 

 flattish ; radii on the inner whorls numerous, more distant and stronger 

 on the outer, end in strong spines on the outer edge of the whorls, pointing 

 outwards ; spines strong, [p. 66] angular, often with a single groove ; 

 transverse striae on the inner whorls, and on the back, numerous, obsolete 

 on the outer whorls ; aperture transverse, rounded, or subquadrate ; 

 diameter 7 J inches 



" This is one of the species noticed by Young, as approaching near 

 to A. armatus, Sow. It is, however, a coarser species, and besides other 

 characters, the outer whorls are narrower in proportion, the radii more 

 distant and stronger, and the spines become very powerful, separated 

 by deep concavities. — L.L. 



" Simpson, 1884, 98, adds after name " Pal.,, pi. xxix " [Wright, 

 Mon. Lias Amm.]. 



Remarks 



Proportions, 198, 22, 23, 60, (thickness to base of spines) ; 

 substeno-, sublepto-gyral ; subextremilatumbilicate. 



Stages, conch, serpenticone ; periphery, 1 ; ornament, 5. 



Whorls are almost squared, and only just in contact. Rugosities 

 on sides strong, broad, wave-like, occasionally sulcated, leading up to 

 large regular spines on edge of a worn and damaged, but apparently 

 smooth periphery (outer whorl). Body-chamber complete, with portion 

 of plain mouth at inner edge — the chamber just over a whorl in length. 



Spines are septate on air-chambers, and so are the first three spines 

 of body-chamber : the septum higher up successively in each. After 

 these the spines appear to contain no septum. 



This is the species described by Simpson in 1855 as A. spicatus, 

 but it differs from the species of 1843 by narrower, thicker whorls and 

 by the stronger rugosities on the sides. From A . hastatus, Young & Bird, 

 this species has whorls thinner and more quadrate, and spines more 

 radially projected. 



Geological position would be presumably L.L.y. 



Result 



Deroceras impavidum, nom. nov. Charmouthian, [armatum zone, 

 Robin Hood's Bay, near Whitby]. 



