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Genus 3. CRANIA, Betzius. 



Animal ; with two large cirr hated appendages forming a pair of 

 spiral arms, curved inwards, unprovided with any apophysary 

 skeleton, but directed towards a concavity of the upper valve, in 

 which it is supported by a central prominence in the lower valve ; 

 mantle extending to the edges of the valves and closely adhering, 

 its margins plain. 



Shell ; inequivalve, inequilateral, squarely orbicular, rather com- 

 pressed, the upper valve being limpet-like ; under valve flat, 

 imperforate, agglutinated to foreign substances ; no hinge or 

 ligament. 



In the two preceding genera of Brachiopods we have seen that it is the 

 natural position of the mollusk to repose upon its back ; the lower valve of 

 the shell is the dorsal valve, and the upper or ventral valve, from which 

 anchorage is obtained, projects over one side, like a beak, for lowering the 

 anchor. In Crania the position of the mollusk is reversed, the ventral 

 valve is undermost, and having no need, or even space, for a tendinous 

 anchor, the valve becomes agglutinated at once to the place of attachment. 

 But it will be observed, that in the genus Orbicula the shell is not so 

 closely adherent, and there is a tendinous muscle of attachment passing 

 through a central fissure of the under valve; and it is conjectured that 

 the same thing obtains in a rudimentary form in Crania in an early stage 

 of its development. The arms, or brachial appendages, of Crania have 

 no internal apophysary skeleton for their support ; they are folded into a 

 pair of spiral coils, which are directed towards the concavity of the upper 

 valve, a limpet-like plate, and, indeed, described as a Patella by Montagu, 

 and they are supported in the concavity of the upper valve by a central pro- 

 minence rising up between them from the lower. This central prominence 

 has somewhat the form of the human nose, and the scars of the anterior 

 adductor muscles above it, regarded as eyes, give to the valve no very un- 

 fanciful resemblance of a face. The under valve of Crania, as with other 

 adherent shells, varies much in thickness according to the nature of its 

 place of attachment. When adhering to branches of coral or any uneven 

 bottom, the animal secretes a thick and solid valve ; when affixed to the fiat 

 surface of shells or stone, the valve is often so thin as to be overlooked. 



The Crania are all of deep-dwelling habits, one species, C. anomala, is 

 dredged in the seas of North Britain and ranges throughout the Scandi- 

 navian waters, the others are from the Mediterranean and seas of India 

 and South Australia. 



VOL. II. 2 B 



