351 



I only know so few of the above-mentioned species from personal observation, 

 I must provisionally refer them all to a single genus Petralia Mac Gillivray; but 

 I do not doubt that this will in time be divided into several. 



In spite of the great variation shown by most of the characters the separate 

 species in this family are so closely connected by many agreements that there 

 can be no doubt about their near relationship. Spines which may appear in a 

 number of 2 — 6 are only found in 6 species, namely, P. magnifica, P. bicuspis, 

 P. tuberosa, P. rectilineata, P. vultur and P- Elleri. On the other hand, they are 

 lacking in P. porosa, which is very nearly related to the two last-mentioned spe- 

 cies. The aperture is provided as a rule with a more or less concave, more rarely 

 straight or slightly convex, proximal margin and its form may be circular (P- 

 undata, P. magnified), quadrangularly rounded (P. bisinuata) or more or less 

 elongated semicircular (P. tuberosa, P. dorsiporosa etc.). Each lateral margin is 

 sometimes more or less distinctly incurved (P. japonica, P castanea). Whilst 

 teeth are quite wanting in P japonica, P tuberosa, P. rectilineata and P. dorsi- 

 porosa, there is a single median tooth in P. castanea, two very small teeth medi- 

 ally on the proximal margin in P- undata and in all the other species a median 

 tooth and two lateral teeth, the latter of which may also be of somewhat differ- 

 ent form. In P. porosa and nearly related species, where they are situated far 

 out to the sides, they resemble in form and position the hinge-teeth in many 

 Smittina species, but they cannot be compared with these as they are placed 

 outside the operculum. In P. bisinuata the three teeth have obviously only arisen 

 by the formation of two incisions in the proximal margin of the aperture, and 

 in this regard an undescribed form from Singapore is of interest, as the concave 

 proximal margin of the aperture is in some zooecia quite entire, whilst others 

 are provided with one or two incisions similar to those found in P. bisinuata. 



The rosette-plates, which are present in fairly large to large numbers, are 

 uniporous only in P. bicuspis, whilst in the other species examined by me they 

 are provided with 3 — 10 pores. 



Of the hitherto known species of this family only four are incrusting (P. bi- 

 cuspis, P. Thenardi, P. rectilineata, P. aviculifera), whilst the others occur in 

 free, one-layered, laminate expansions, and it may be considered as a good fam- 

 ily character that the basal wall in all these species is provided with pores, 

 which otherwise appear only very exceptionally on the basal wall of free, one- 

 layered colonies within the division Ascophora. Whilst these pores occur scat- 

 tered over the whole of the basal wiall in P. undata and P. japonica, in all the 

 other species they appear in one or more, rarely (P- dorsiporosa) several pore- 

 chambers, which are situated at the distal end of the basal wall. In the unde- 



