250 NOTES ON THE BEITISH JURASSIC BRACHIOPODA. 



transverse, especially in the young age, sometimes provided 

 with lateral wings, as in Spirifer or Megaihyris (Argiope). 

 Surface ornamented with thick longitudinal costre, generally 

 few in number, acute, or rounded, but always very promi- 

 nent, crossed by strong transverse ridges or lines of growth, 

 which produce an ornamentation broadly reticulate and 

 elegant. Beak large and thick, truncated by a triangular 

 foramen in young age, more or less rounded in the adult, 

 sometimes in the middle of a flat area as in Spirifer; 

 deltidium always slightly pronounced, remaining often 

 incomplete in adult age. Beak of the ventral valve showing 

 always in the interior well-marked rostral septa. In the 

 interior of the dorsal valve the cardinal platform is slightly 

 developed, and rests upon a median septum more or less 

 pronounced, which reaches half the total length of the 

 valve. The brachial apparatus takes its definite form only 

 in adult age, the Megerliform stage persisting in Ismenia 

 much longer than in the other groups which pass through 

 various stages in the metamorphosis of the apophysary 

 system" (12, p. 263). 



Ismenia first makes its appearance in the Liassic rocks, 

 where it is represented by I. perrieri, and I. suessi ; E. Desl. 

 sp. Both of these extremely rare and beautiful shells were 

 found by Moore in the Middle Lias {Margaritatus zone) at 

 Ilminster. 



On the Continent, Ismenia is also met with in the Inferior 

 Oolite, Fullers' Earth, Oxfordian and Coral lian rocks. It 

 is also found in existing seas. 



CEhlert makes Ismenia a sub-genus of Bayle's genus 

 Milhlfeldtia. 



