463 ADDITIONAL NOTES 



AcROSALENiA PARVA, Wright, iiov. sp. 



Ambulacral areas with two rows of small marginal tubercles; inter-ambulacral tubercles 

 large at the equator, and small on the upper and under surfaces ; mouth opening wide, 

 indistinctly decagonal, spines long and hair-like. 



Dimensio7is. — Transverse diameter of the largest test two lines ; height unknown. 



Description. — The ambulacral areas of this Acrosalenia £ire moderately wide with 

 two rows of small, perforated tubercles on the margins of the areas, a smaller 

 tubercle alternating with a larger one throughout the row. The poriferous zones are narrow, 

 and the holes large and distant from each other. The inter-ambulacral areas are wide, 

 the plates have a single row of primary tubercles near their zonal sides, the tubercles near 

 the equator are large, and raised on prominent bosses with crenulated summits ; the areola 

 are narrow, and confluent above and below, a semicircle of microscopic tubercles encircles 

 the boss on its zonal side, and a zig-zag line of tubercles occupies the middle of the area 

 which forms similar crescents on the sutural side of the areolae ; all these microscopic 

 tubercles are perforated, a fact which can only be ascertained by the aid of a microscope 

 with a half-inch object glass. The crenulations on the summit of the large bosses, when 

 seen in profile with the microscope, resemble a circle of beads around that prominence. 



The mouth-opening is wide, and indistinctly decagonal. The long, fine, and hair-like 

 spines are scattered in profusion over the surface of the slab. When examined with a half- 

 inch'object glass, their surface is seen to be covered with sharp longitudinal lines, having 

 an indistinctly undulated edge. 



Affinities and differences. — This species differs from Acrosalenia minuta of the Lower 

 Lias in having the ambulacral areas much better defined, and the tubercles of the inter- 

 ambulacra larger and more prominent. Although a very small urchin, its generic 

 characters are well marked ; in the general neatness of its test it resembles some of the 

 young forms of Acrosalenia spinosa, Ag., from the Cornbrash ; this urchin affords another 

 of those examples, so numerous among the Echinodermata, that the earliest forms of genera 

 are, in general, those in which the typical characters of the group are best developed. 



Locality and Stratip-aphical position. — This small urchin was found by Mr. Tomes, 

 who has kindly communicated it for description in the Lower Lias of Warwickshire, in 

 the zone of Ammonites ohtusus, it was associated with Ammonites Birchii, Sow., and has 

 numerous small Gasteropoda and Conchifera imbedded with it on the same slab. It 

 is the oldest Acrosalenia that has yet been found in the Lov/er Lias. 



