474 MR. HERBERT L. HAWKINS ON 



sufficient to mask the plan of the tuberculation. Irregularity 

 generally consists of either the suppression of a member of 

 a series on one plate (a feature often seen, even in the primary 

 row, in Echinus), or its replacement by two tubercles. The 

 Holectypoicia are peculiar among Irregular Echinoids in retaining 

 throughout life the unituberculate plates and the vertical arrange- 

 ment of the tubercles. Only the Echinoneidse show any similar 

 plan, and in them it is very much obscured by irregularity 

 of development. Some forms of Pygurus (e. g., P. blumenbachi) 

 have one or more unituberculate plates at the adapical extremities 

 of their interambulacra, but this retardation of development is 

 obviously due to the considerable narrowing of the plates caused 

 by the expansion of the ambulacral petals. However, most, if 

 not all, of the Irregular Echinoids whose post-larval development 

 has been studied, show a unituberculate stage. This is notably 

 the case in Echinolavipas (see Agassiz, 30). 



The tubercles of the additional series which develop on the 

 interradial tracts are at first situated each in the middle line (in 

 a vertical sense) of the plates. This results in a transverse line 

 of tubercles extending between the main series and the inter- 

 radial suture. The concentric arrangement thus caused charac- 

 terizes Pygaster sens, lat., Holectypus sens, lat., and Discoidea. 

 It seems somewhat irregular in the case of Pileus, and is definitely 

 absent in Aiiorthopygus and Gomdus. In these two genera the 

 tubercle series of the interradial tracts appear near to the adapical 

 and adoral margins of the plates alternately, thus giving an 

 oblique arrangement (sloping interradially and adorally) to the 

 tubeicles in the complete interambulacrum. A similar arrange- 

 ment to this afiects the adradial tubercle-series in all the genera 

 of the group. 



The oblique setting of the tubercles results in a much more 

 uniform and packed tuberculation over the whole area than 

 exists when the interradial series are transverse. The closeness 

 of the arrangement is increased by the doubling of many of the 

 tubercles in Conulus; so that, instead of two tubercles, three or 

 even four are concerned in the composition of the oblique line on 

 each plate. The complexity of arrangement, coupled with a 

 homogeneity of character, of the tubercles, which was thus slowly 

 obtained during the course of evolution of the Holectypoida, was 

 rapidly developed, and carried to a further degree, by the earliest 

 of the non-Holectypoid Echinoids. The species of Galeropygvs 

 from the Lias show typically the bewildering profusion of small 

 tubercles which characterize the interambulacra of all the 

 ISTucleolitidfe, " Cassidulidae," Clypeasti^oida, and early Spatangida?. 

 Only the Echinoneidae seem to preserve a C'o5i^tZ^6s-character in their 

 tuberculation, and in them it becomes so irregular as to be hardly 

 appreciable except in the newest formed plates. 



The peculiar sunken supernumerary tubercle of some of the 

 adapical interambulacrals of Holectypus depressus from the Corn- 

 brash recently described (Hawkins, 67) is without a parallel 



