FROM THE LIAS. 231 



them satisfactorily it is necessary to examine the test with a microscope under an inch 

 object-glass. 



The poriferous zones are extremely narrow; the pores are small, there being four pairs 

 opposite each tubercular plate ; the septa are slightly elevated, and form a microscopic 

 moniliform line between the pores (fig. 2 e). 



The inter-ambulacral areas are wide (fig. 2 6, c), with two rows of primary tubercles, 

 from nine to ten in each row; they are situated near the zonal sides of the plates, and have 

 a wide miliary zone between them ; the bosses have deeply crenulated summits, and the 

 tubercles are small and widely perforated ; well-defined areolas encircle the bosses (fig. 2 e), 

 which are confluent above and below ; the miliary zone consists of an elevated band, 

 composed of from four to six rows of small, unequal-sized granules (fig. 2 e), which extend 

 from the peristome to the disc. When viewed with the naked eye, at the equator, this 

 tiny urchin appears to possess only ten rows of tubercles, nearly equidistant from each 

 other (fig. 2 a), but when examined with a microscope its true structure is seen ; the 

 extreme narrowness of the ambulacral areas, with their close, alternate rows of microscopic 

 tubercles, and the width of the miliary zone, with its unequal-sized granules, ahke 

 contribute to make the deception almost complete (fig. 2 d). 



The opening for the disc is nearly one half the diameter of the test, but I have never 

 seen the trace of a plate in any specimen I have examined (fig. 2 c). 



The mouth opening is small, about one third the diameter of the test, and lies in a 

 concave depression (fig. 2 d) ; the peristome is feebly notched, and the lobes are of unequal 

 size (fig. 2 d). 



The most remarkable portions of the structure of this tiny fossil are the spines, which, 

 in some crushed specimens, are preserved in situ between the laminae of the Lias shales ; 

 they are long, slender, and hair-like, with a well-developed head ; on some slabs these 

 spines resemble so many fine bristles, laid down in all directions, upon the surface of the 

 laminated Oxynotus shales ; in one crushed test, four tenths of an inch in diameter, the 

 spines measured upwards of an inch in length. 



Jfmities and differences. — The smallness of the test, and the length and hair-like 

 character of the spines, are sufficient to distinguish Acrosalenia minuta from all other Lower 

 Lias urchins. In the shales of the Upper Lias, there is another small urchin with long, 

 hair- like spines ; but I have never yet succeeded in obtaining a specimen in sufficient 

 preservation to enable me to institute a comparison between it and Acrosalenia minuta ; 

 the Upper Lias form reminds me very much of Cidarites crinifera, Quenstedt, from the 

 " Posidonienschiefer von Pliensbach bei Boll in Wiirtemberg," a bed of the Upper Lias. 



Locality and Stratigrapliical position. — Acrosalenia minuta was found in the laminated 

 Oxynotus shales of the Lower Lias at Lansdown, Cheltenham, and likewise at Gloucester, 

 whilst excavating the same bed to form a new dock. It was associated in both places with 



