298 H. L. Hau-kins — A New Fibularia from Nigeria. 



a distance corresponding closely with the length of tiie ambulacral 

 petals. Beyond that region a very sudden increase in declivity appears, 

 and the walls of the test become almost vertical. It is impossible to 

 ascertain where the adoral surface begins, as the margin of the test is 

 perfectly rounded. The maximum diameter is at a level approximately 

 two-thirds of the distance from the apex to the base. In the forms 

 which approach most nearly to a globular shape, there is a tendency 

 to a slight flattening of the adoral surface over an area whose radius 

 is the line from the centre of the peristome to the adapical margin 

 of the periproct. Tliere may even be a slight concavity in this 

 region. But in the elongated forms no such flattening is developed, 

 so that the specimens will not stand on the adoral surface, but lie on 

 one side. 



An important feature which is shared by all the specimens is the 

 carinated character of the ambulacra, and a consequent alternation of 

 concavity and convexity in the ambital outline, comparable with that 

 of many species of Clypeaster. (This is shown, to a slightly exaggerated 

 degree, in PI. XVI, Yi^. 4.) The interambulacra are slightly sunken, 

 chiefly along their median sutures, and the perradial parts of the 

 ambulacra are equally depressed. But on the ambulacral areas, less 

 than half-way between the interradial and perradial sutures, a more 

 or less prominent elevation passes down from the ends of the petals 

 almost to the peristome. The carinee are more prominent in the 

 elongated forms, but are quite obvious even in the most spherical. 



The Ambulacra. — The ambulacra are petaloid for from eight to 

 fourteen of their plates from the apex (according to the size of the 

 specimens), and increase considerably in width below the petaloid 

 region. They become rapidly narrower below the ambitus, but occupy 

 most of tlie peristomial margin. The width of the ambulacra is to 

 that of tlie interambulacra (measured at the ambitus) as 5 : 3. 



The posterior petals are very slightly shorter than the others, and 

 in elongated specimens the lateral ambulacra are perforce shorter than 

 the anterior. The plates are all primaries, but are often cuneiform. 

 The petals are in a few instances slightly, but definitelj', inflated, but 

 are usually flush with the test surface. The number of pores in the 

 petals shows an increase, following the growth in size of the individuals, 

 but slight variations in the number (of quite an irregular type) often 

 occur in the petals of a single specimen. The pores are rounded or 

 elliptical, no regular difference being shown between those of the 

 outer and inner series. They are very large in proportion to the 

 size of the test. The two pores of a pair are set obliquely, sloping 

 interradially and adorally, and are joined by a shallow groove of 

 a width equal to the diameter of the pores. The obliquity of the 

 pore-pairs becomes more intense as they are traced outwards from the 

 apex, so that the inner row of pores is more closely packed than 

 the outer. The last three or four pore-pairs on each half area pierce 

 a more perradiad part of the plate than the uj^per series. This gives 

 a tendency to a closing of the ])etals, a feature which is more apparent 

 than real, as it is exaggerated by the extreme obliqueness of the distal 

 pore-pairs. 



As far as I have been able to ascertain, there are no other pox'es in 



