TEEMACYSTIA. 173 



Distribution. Upper Green Sand : Warminster, Wiltshire ; Essen, Rhenish Prussia 

 {Steinmann). 



Tremacystia siphonioides, Mich. sp. (Plate XXXIV. fig. 2.) 



1840-47. Thalamopora siphonioides, Midi. Icou. Zooph. p. 210, t. 53. f. 9 a, b. 

 1882. Sp/i(eroca'lia Michelinii, Steinmann, pars (non Simonow.), Neues Jahrbuch, Bd. 2, 

 p. 162, t. 7. f. 4 a, 6 (nonf. 4). 



Subcylindrical sponges, from 13 to 26 mm. in height, and about 7 mm. in diameter, 

 with a truncated summit and a cloacal aperture about 1-75 mm. in width. Growing 

 singly or in small groups on an extended base. There are from 2 to 4 segmental 

 chambers, from 5 to 6 mm. in height, in the individual sponge. Slight constrictions 

 are shown on the outer surface, marking the commencement of fresh chambers. 

 The walls are from -2 to -3 mm. in thickness. The interior is not shown in the 

 specimens I have examined ; but according to Michelin there is a continuous cloacal 

 tube. The walls are perforated by subcii'cular apertures about "2 mm. in width. 



I have not been able to ascertain the internal structure of the fibre ; but in a 

 specimen from La Heve there is on the outer surface a dermal layer of three- or 

 four-rayed spicules similar to those on the surface of T. iJ'Orbignyi. 



This species differs from T. UOrhignyi in the cylindrical form of the sponge and 

 its individual segments. It is distinguished from T. Michelinii, Simonow., by its 

 central cloacal tube and the single wall in the floor of the chambers. 



Distribution. Craie Chloritee : La Heve. Upper Green Sand: Warminster, Wilt- 

 shire. 



Tremacystia Michelinii, Simonowitsch, sp. 



1871. Tlialamopora Michelinii, Simonow. Beitr. z. Kennt. der Bry. des Essener Grlinsandes, 

 p. 31, 1. 1. f. 2 a, b, c. 



I have not seen an undoubted example of this species ; but according to the 

 detailed account and figures of Simonowitsch, the sponges consist of a series of four 

 to five depressed spherical chambers, gradually increasing in size. Each chamber 

 has a separate base, which rests on the roof of the subjacent chamber, so that there 

 are double walls between them. No cloacal tube is present, but each chamber com- 

 municates with that above by a central aperture with a raised margin. The walls are 

 relatively thin, and perforated by circular and elliptical apertures. 



Relying on the accuracy of this description, there appears to me no doubt of the 

 distinctness of this species from the two preceding. The double partitions of the 

 chambers and the absence of a cloacal tube clearly mark it off both from T. DOr- 

 bigniji and T. siphonioides. 



Distribution. Cenomanian: Essen {Simonowitsch). 



