T. H. Withers — Verruca from tJte Horivich Chalk. 105 



Distriiution. — Maestrichtian and Upper Senonian : St. Pierre, 

 Duchy of Limbourg, HoUaud. Maestrichtian : Sichen, Belgium. 

 Upper Senonian : Frere, near Tongres, Belgium. Upper Senonian, 

 Belemnitella 7nucronat a- zone : near Norwich, Noifolk, and I. of Riigen. 



The largest example in the present series is attached to an oyster 

 and measures 4"2 mm. in its greatest diameter from tlie base of the 

 rostrum to that of the carina, while the specimens attached to the 

 shell oi Echinocorys range from 1-2 mm. to about 4 mm. in the same 

 dimensions ; the two examples with the opercular valves in position 

 measure respectively l'2mra. and 2-1 mm. Darwin's figure of the 

 Norwich specimen was enlarged to five diameters, and since this 

 figure measures 24 mm. in breadth, the specimen must have been 

 somewhat larger than the largest of the present series. I have 

 already pointed out that it seems to be a matter of chance whether 

 the moveable opercular valves are on the right or the left side, 

 and the four well-preserved examples show this quite clearly. 



Fig. 1. Verruca prisca,i.'Bosq\xei. Upper Senonian : Belemnitella viucronata- 

 zone, near Norwich. Complete shell (attached to Echinocorys scutatus). 

 (a) View showing the moveable opercular valves, which are on the left 

 side ; the rostrum and carina have three unequally-developed interlocking 

 ribs. (6) Opposite view of same specimen showing fixed scutum and 

 tergum. x 15 diam. 



Fig. 2. Id. {a) Specimen, attached to Ostrea semiplana, with the four valves of 

 the shell united, but in which the moveable opercular valves are missing. 

 The rostrum is to the left hand, therefore the moveable opercular valves 

 must have been on the right side ; there are three equally-developed 

 interlocking ribs to the rostrum and carina. (6) Opposite view of same 

 showing the fixed scutum and tergum. x 7'5 diam. 



r. rostrum ; c. carina ; s. moveable scutum ; s. fixed scutum ; t. move- 

 able tergum ; t' . fixed tergum. 



In the specimen attached to the oyster (rig. 2), and in one 

 attached to the Eehinoeorys, the opercular valves must have been 



