240 ROBERT TRACY JACKSON ON ECHINI. 



disc through the plane III, 5 is 4 mm. This measurement includes from the adoral border 

 of ocular III to the aboral tip of the youngest plate of interambulacrum 5. The apical disc 

 measures proportionately 36% of the diameter of the test. This proportion is about the 

 same as in the modern Cidaridae and is relatively larger than in other known Palaeozoic 

 species (pp. 87, 104). Oculars are large, forming a closed ring, thus covering the ambul- 

 acra and inter-ambulacra completely, a unique character. Genitals are small and shut out 

 from contact with the interambulacra by the continuous character of the oculars (p. 87). 

 The periproct has many small angular plates filling the area. 



This species differs from B. pahleni in that there are perforate tubercles on both ambulacral 

 and interambulacral plates, whereas in pahleni they are absent in the interambulacrals. It 

 also differs in that the genitals are wholly dorsal to the oculars instead of part of them separating 

 the oculars as in pahleni. Boihriocidaris archaica differs from globulus in that the oculars form 

 a closed ring, whereas in globulus the oculars are separated by the genitals which meet the 

 interambulacra of their several areas. 



Lyckholm Schicht, Ordovician, Hohenholm, Island of Dago, Russia. The type and 

 only known specimen is in the Berlin Museum as above stated. 



In the morphological part of this memoir much attention is given to Bothriocidaris as a 

 primitive type, and the consideration is largely based on the study of this species. The ambula- 

 crum is a feature of particular interest. The whole of ambulacrum IV measured on the curve 

 from the oral termination to the ocular is 15 mm. in length. The two ventral rows of ambula- 

 cral plates are considered as peristomal, from which the basicoronal plates of the corona are 

 not sharply defined. In the corona proper, and excluding the peristome, there are eight ambu- 

 lacral plates in each column of each area excepting Ilia, in which there are nine plates, as a 

 very young plate is in contact with the ocular, but so low that it does not quite reach the inter- 

 ambulacrum. Pore-pairs are superposed in the middle of each plate, a condition known in the 

 adult of regular Echini only in this genus (text-fig. 22, p. 70). At the mid-zone there are four 

 perforate tubercles to a plate, or in some plates fewer; when fewer at this area, the differ- 

 ence may be due to loss from erosion. Ventrally and dorsallj' there are fewer tubercles as a 

 character. In the sixth plate from the base in ambulacrum II o, the tubercles were not seen, 

 but are filled in by comparison with adjacent plates, otherwise the tubercles of ambulacral 

 plates are figured as observed. The hexagonal shape and superposed pores are like the young 

 of later Echini (pp. 54, 57). 



Interambulacrum 3 measures on the curve 12 mm. in length. In each interambulacrimi 

 there are nine plates, and the basicoronal plate of each area is a pentagon, the ventral apex of 

 which impinges on the second row of peristomal ambulacral plates (compare Plate 2, fig. 1; 

 Plate 3, figs. 9, 10). The tubercles of these plates are few in number, but striking. There 

 is a single tubercle on the basicoronal plate in each area, also a single tubercle on the second 



