INOCK RAMUS. 



319 



mteniiLMliate between tliis type and those witli runndetl folds occur. It may 

 be convenient to adopt lor this variety the name /. TjH tmi rrJ:'i ww. ]V('hsfi'ri. It 

 appears to occur mainly in the zone of Micraster cor-testadiimrium. 



]. tniiliilnfns, ]\[antell (PI. LIII, fio^. 3), resembles small forms of I. BroiKjinarll, 

 Sowei'l)y, but the folds are much smaller, more numerous, and less conspicuous, 



an 



d f 



le shell IS thicker tlian usual. 



i. 





Fig. 77. — Iiiocohhiks Lmiinrchi var. Citvieri, Sow. U|>|)( r Chalk (zone of Holiisler ji/miiis), Si\:ifTli:iiii. 

 Niiil'ulk. Norwich Mtiscuiii. I.oft valve. x J. 



Some small forms (IM. Id 1 1, figs. 4— G), found in the .Middle Chalk and in 



the zone of Iluldstcr jihiniis, which may 1k> named /. Ldinnrrkl var. apicdlix,^ have 



neai-ly eipial valves, more prominent ami more distinctly incurved umbones, with 



the folds indistinct or al)sent, but these forms pass into others with distinct folds. 



In some of these small forms the umbo is curved anteriorly (Plate LIII, tiu^. -I). 



' An I'Xiimiik- of tlii.s I'l-am the Chalk Rock was figured in tlie ' Quart. Ji>urn. Geol. Soc," vol. liii 

 (18St7), p. 381, pi. .\xvii. fi-. 1:;. 



