112b YORKSHIRE TYPE AMMONITES— II July 



112. AMMONITES SEMICOSTATUS, Young & Bird 

 (Plate CXII) 



Original Description 

 [Young & Bird, 1828, pp. 257, 359.] 



" [P. 257.] No. 10, PI. xii, is a neat little shell from the lias bands, 

 having a low keel, with a slight furrow on each side. The inner whirls 

 are entirely smooth ; the outer whirl (and sometimes part of the next) 

 is marked with sharp straight ribs, ending abruptly, so as to make a 

 sharp knob at the back, with sometimes a slight bend forward. The 

 aperture is round ; the shell commonly less than half an inch broad. 

 This shell we may call A. semicostatus. It often occurs in large clusters 

 in the lias nodules. — This ammonite resembles A. Smithi, Sowerby, 

 Tab. 406 ; which appears to occur in the same nodules. 



" [P- 359] Plate xii ... . Fig. 10. Ammonites semico status. 

 Lias bands." 



Remarks 



Proportions, 39, 28, 24, 50 ; Subplaty-, subleptogyral, perlatum- 

 bilicate. 



Stages, conch, serpenticone ; periphery, 5 ; ornament 1, 2, 3, 4, 5. 



The conch is truly serpenticone, being decidedly polygyral : there 

 are at least 6 whorls. All the inner whorls, with their rather strong, 

 though ill-defined, rounded inner margin, with their nearly circular section, 

 slow increase, and striate markings, look much like a coiled worm. The 

 feebly striate stage, looking like a smooth stage, unless viewed with a 

 lens, persists nearly to the commencement of the last whorl ; but, at 

 distances of about each half -whorl, two or three obscure median bulgings 

 may be detected. The striae strengthen and pass into subcostae about 

 the beginning of the last whorl, and the latter soon become costae, each 

 of which quite quickly develops a small tubercle on the peripheral border. 

 These costae are at first laterally rursiradiate, but towards end of whorl 

 have become versiradiate. Keel and furrows are distinct at beginning 

 of last whorl — they thus precede the costate stage — and they strengthen 

 with the whorl. Growth-lines sweeping forward pass over keel so 

 distinctly as to make it appear faintly crenulate. The keel is of somewhat 

 low relief. Aperture oblong. 



The specimen now illustrated, entered as A. semicostatus in Whitby 

 Museum register, is presumed to be the holotype from its agreement 

 with the original description : the remark " the shell commonly less 

 than half an inch broad " being taken as a general statement, not neces- 

 sarily applicable to the type. Some other statements also indicate that 

 more than one specimen was used in the description ; but the main remarks 

 fit this example very well. The figure gives no assistance, beyond the 

 fact that it shows a species of this kind, drawn with its characteristic 

 ribs reversed. 



Genus, Arnioceras ; Family, Arietidae. 



Geological position : Simpson (1884, p. 133) says " L. L. 25. R. H. 

 Bay " ; but enters it in 24 (p. xxn). 



Result 

 Arnioceras semicostatum, Young & Bird sp., 1828, Sinemurian, 

 semicostatum zone, [Robin Hood's Bay], near Whitby. 



