HIPPURITIDiE. 203 



The}' are the most problematic of all fossils ; there are no 

 recent shells which can be supposed to belong to the same 

 family; and the condition in which they usually occur has 

 involved them in greater obscuritv. The characters which 

 determine their position amongst the ordinary bivalves are the 

 following : — 



1. The shell is composed of three distinct layers. 



2. They are essentially uns^'mmetrical, and right-and-left 

 A^alved. 



3. The sculpturing of the valves is dissimilar. 



4. Tliere is evidence of a large internal ligament. 



5. The hinge-teeth are developed from the free valve. 



6. The muscular impressions are two only. 



The outer layer of shell in Radiolites consists of prismatic 

 cellular structure ; the prisms are perpendicular to the shell- 

 laminse, and often minutely subdivided. The cells appear to 

 have been empty, like those of Ostrea. The inner layer, which 

 forms the hinge and lines the umbones, is subnacreous, and 

 very rarelj^ preserved. It is usually replaced by calcareous spar, 

 sometimes by mud or chalk, and very often it is only indicated 

 by a vacuity between the outer shell and the internal mould. 

 The inner shell-layer is seldom compact, its lamellte are 

 extremel}^ thin, and separated by intervals like the water- 

 chambers of Spondjdus; similar spaces occur in the deposit, 

 filling the umbonal cavity of the long-beaked oysters. 



The chief peculiarity of the Hippuritidse is the dissimilarity in 

 the structure of the valves, but even this is deprived of much 

 significance by its inconstancy. The free valve of Hippurites 

 is perforated by radiating canals which open round its inner 

 margin, and communicate with the upper surface by numerous 

 pores, as if to supply the interior with filtered water ; possibly 

 they were closed by the epidermis. In the closely allied genus 

 Radiolites there is no trace of such canals. 



The teeth of the left, or upper, valve are so prominent and 

 straight, that its movement must have been nearl}'' vertical, 

 for which purpose the internal ligament appears to have been 

 exactly suited by its position and magnitude ; but it is prob- 

 able that, like other bivalves, they opened to a very small extent. 



HiPPURiTES, Lamarck, 1801. 



Etym. — Adopted from old writers, "fossil Hippuris," or 

 Horse-tail. Syn. — Batolites, Raphanistes, Montf. 



Dialr. — ^Fossil, 30 sp. Chalk ; Bohemia, Tyrol, France,, 

 Spain, Turkey, S^a-ia, Algeria, Egypt. H. toucasianus (cxviii, 

 2t, 28). H. sulcatus, Defrance (cxviii, 29, 30). 



Shell very inequivalve, inversely conical, or elongated and 

 cylindrical; fixed valve striated or smooth, with three parallel 



