HIPPURITID. 203 
They 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 obscurity. 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 unsymmetrical, and right-and-left 
valved. 
3. The sculpturing of the valves is dissimilar. 
4, There 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 Radiclites consists of prismatic 
cellular structure ; the prisms are perpendicular to the shell- 
lamine, 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 rarely 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 lamelle are 
extremely thin, and separated by intervals like the water- 
chambers of Spondylus; similar spaces occur in the deposit, 
filling the umbonal cavity of the long-beaked oysters. 
The chief peculiarity of the Hippuritide 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 nearly 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. 
Hiprurites, Lamarck, 1801. 
Etym.—Adopted from old writers, “fossil Hippuris,” or 
Horse-tail. Syn.—Batolites, Raphanistes, Montf. 
Distr.—Fossil, 30 sp. Chalk; Bohemia, Tyrol, France, 
Spain, Turkey, Syria, Algeria, Egypt. H. towcasianus (exviii, 
27,28). H. sulcatus, Defrance (exviii, 29, 30). 
Shell very inequivalve, inversely conical, or elongated and 
cylindrical; fixed valve striated or smooth, with three parallel 
