67 



dropped to show the hinge of parallel grooves containing the liga- 

 ment. 



Genus 5. CRENATULA, Lamarck. 



Animal ; undescribed. 



Shell ; thirty nearly equivalve, smooth, mostly concentrically striated, 

 oblique ; hinge lateral, linear, crenulated, crenules set in a row 

 along the margin, callous, rather hollow, receiving the ligament. 



The Crenatida, of which eight species are now known, are distinguished 

 by a marked speciality of character. They cannot be said to run into any 

 of the proximate genera. Their shells are of a delicate horny substance, 

 and of a curiously oblique-quadrate or obliquely-ovate growth, the hinge 

 being set on a prolonged lateral margin in a row of jutting hollow crenules. 

 All the species are from the Red Sea, excepting one of very distinct typical 

 character, C. flammea, from New California. Like the Vulsella, they live 

 imbedded in sponge. 



Species. 



1. avicularis, Lam. 4. folium, Gray. 7. picta, Gmel. 



2. bicostalis, id. 5. Mytiloides, Lam. 8. viridis, Lam. 



3. flammea, Reeve. 6. nigrina, id. 



Figure. 



Crenatula avicularis. PL 29. Tig. 169. Shell, with the upper valve 

 dropped to show the jutting ligamentary crenules. 



Family 4. MYTILACEA. 



Shell ; more or less narrowly elongated, equivalve, but very un- 

 equal-sided, of horny Jibro-laminar substance, sometimes smooth, 

 sometimes prickly scaled ; animal ajjixed by a byssus. 



The byssus-spinning Mussels are distinguished from the byssus-spinning 

 Oysters by having a narrower more elongated shell, of lighter more horny 

 substance ; and the animal commences to show the lobes of its mantle 

 united, with a development of siphonal orifices in front for the transmission 



