172 CALCAEEOUS SPONGES, 



the perforated walls of a single layer of fibre, and the similarity in the spicular 

 structure. 



Tremactstia D'Orbignyi, Hinde. (Plate XXXIV. figs. 1, 1 a-\ o.) 



1882. Verticillites D'Orbignyi, Hinde, Ann. & Mag. Nat. Hist. 5th ser. vol. x. p. 192, 1. 10. 



f. 1, 2, 7, 8, and t. 11. f. 1-24. 

 1816. Alcyonite, W. Smith, Strata identified, t. 6. f. 12. 

 1882. SphcerocceUa Michelini, Steinm. pars (uon Simonowitsch), Neues Jahrhuch, Bd. 2, 



p. 162, t. 7. f. 4 (uon 4 a, b). 



Small club-shaped sponges, from 16 to 23 mm. in height, either simple or with 

 lateral branches ; growing singly or in small groups from a common extended base. 

 The individual forms consist of a series, generally four to six in number, of sub- 

 spherical chambers, from 2 to 4-5 mm. in height and from 4 to 6'5 mm. in width. 

 The summit chamber is conspicuously larger than those beneath, and measures from 

 6 to 9 mm. in height and from 9 to 14 mm. in width. The floor of each chamber 

 is formed by the depressed dome-shaped roof of the chamber below it, and thus there 

 is only a single partition-wall between the respective chambers. A central cylindrical 

 tube, about 2 '25 mm. in diameter, connects the chambers, but I have not been able 

 to ascertain whether it is continuous throughout. 



The walls are from -16 mm. to -2 mm. in thickness ; they are perforated by 

 numerous minute circular or subcircular apertures, about -25 mm. in width and 

 the same distance apart. The outer surface is smooth. The fibre of the walls 

 is mainly composed of elongated, curved, filiform, three-rayed spicules, disposed 

 generally in the direction of the fibre, parallel with each other, and in close contact. 

 These spicules are from -15 to --3 mm. in length, and -006 mm. in thickness. The 

 basal ray is very slightly developed, and usually appears as a minute projecting knob 

 in the centre of the spicule. The outer surfiice of the fibres has a dermal layer of 

 three- and four-rayed spicules much stouter and larger than the filiform ones beneath. 

 The longest ray of the largest of these dermal spicules which I have yet met with 

 measures '3 mm. in length and '07 mm. in width. There are numerous gradational 

 forms between the dermal and the filiform spicules. I have not seen any simple 

 uniaxial spicules in the fibre. 



This species is distinguished from Tremaci/stia {Thalamopora) sij^honioidcs, Mich., 

 by the spherical form of the chambers and its slightly thinner wall. From Trema- 

 cystia {Thalamopora) MichelinU, Simonow.*, it differs in possessing a central siphonal 

 tube, and by the absence of a separate basal floor to each of the chambers. Its 

 form, slender walls, and the circular perforations in them distinguish it from 

 T'remac7jstia ( Verticillopora) anastomans, Mant., and its allied species in the Lower 

 Green Sand. 



• Beitr. zur Kennt. der Bry. des Esaener Griinsandes, p. 31, t. 1. f. 2 a, b,c. 



