242 W. J. SOLLAS ON PHARETROSPONGlA STRAHANI. 



16. On Pharetrospongia Strahani, Sollas, a fossil Holorhaphi- 

 dote Sponge from the Cambridge " Coprolite" bed. By W. 

 J. Sollas, Esq., B.A., P.G.S., Scholar of St. John's College, 

 Cambridge. (Read December 20, 1876.) 



[Plate XI.] 



In the collection of Cambridge-Greensand fossils exhibited in the 

 Woodwardian Museum, Cambridge, may be seen some rather large 

 and irregular fossil sponges bearing the name of Chenendopora, sp. 

 These are the sponges referred to under this name by Mr. Bonney, 

 in his Text-book of Cambridgeshire Geology (p. 41), and included, 

 also with the same generic designation, by Mr. Jukes-Browne * in 

 the lists of fossils given by him at pages 311 and 313 of his paper 

 " On the Relations of the Cambridge Gault and Greensand." The 

 responsibility for the name rests, however, solely with the Museum 

 authorities, the authors cited having simply quoted from the labels 

 attached to the specimens in the Museum, the correctness of 

 which they naturally took for granted. Why the sponge is named 

 " Chenendopora " I do not know ; certainly it does not possess the 

 characters of that genus as founded by Lamouroux, nor does it cor- 

 respond with any of the subsequently described species of the genus 

 with which I am acquainted ; it is, though long known to Cambridge 

 collectors as an abundant form, an undescribed species, and, from the 

 very important features it presents, merits a detailed investigation. 



Outer Form. — The Sponge consists essentially of a somewhat thick 

 plate, which, by folding in various ways, and by uniting with itself 

 wherever its sides are folded into contact, gives rise to a great 

 diversity of forms ; thus it is sometimes merely a curved plate, 

 sometimes more or less cup-shaped, while often it is contorted into 

 an intricate pattern, which is made still more intricate by the ana- 

 stomosis of the plate at every point of contact. 



The Plate. — This in its present condition is a solid mineral plate, 

 the two faces of which are, on the whole, parallel to each other and 

 connected by rounded edges ; its thickness is constant in the same 

 specimen, " or nearly so, and varies only within narrow limits in 

 different specimens ; numerous measurements give i of an inch for 

 the mean thickness. 



Prom the constancy in thickness of the plate we may infer that it 

 increased chiefly by additions to its edges ; thus its growth was 

 marginal. 



The Sponge. — The plate, by the more rapid growth of one surfaee, 

 becomes curved, sometimes to such an extent as to form bowl-shaped 

 or cup-like sponges ; in this manner the general curvature from 

 a to g of the specimen represented in PL XI. fig. 1, has been pro- 

 duced. The diagram, Pig. 1, is a section showing this curve. 



If the rate of growth of the peripheral portions progressively 



* Quart. Journ. Geol. Soc. vol. xxxi. (1875). 



