146 Dr. H. Woodward — A Neio Ciry'i'pede from the Chalk. 



Darwin, in describing the genus Pyrgoma,^ says: — "The shell 

 consists of a single piece, generally without suture, even on the 

 internal surface ; and this is the case, at least, in P. anglicum, in 

 extremely young colourless examples : nevertheless, in some speci- 

 mens of this very species, and of P. conjngatmn, there were traces of 

 two, but only tivo, sutures on the sheath, one on each side towards its 

 carinal end. The shell is often much depressed or actually flat ; in 

 P. anglicum, however, the shell is steeply conical. The outline is 

 rather oval. The surface is furnished with more or less prominent 

 ridges, radiating from the orifice, which is oval and small." (See 

 PI. VIII, Fig. 5.) '-The shell," he adds, " is unusually thick." 



"The basis, in all the species, is more or less regularly cup-formed 

 or sub-cylindrical. In P. grande it penetrates the coral (on which 

 it is fixed) to a surprising depth ; but this is not the case with 

 P. anglicum, in which the basis is generally exserted, as it is in 

 a slight degree in P. grande." 



Of the opercular valves in the Chalk species, so important and 

 essential in the study of any of the Cirripedia, we still remain in 

 ignorance. I should not, therefore, have ventured to reopen the 

 previous description of the so-called ' Pyrgoma cretacea,' had it not 

 happened that a new and important light has been thrown upon it, 

 quite unexpectedly, through the discovery in the Chalk of Thanet of 

 a second specimen by my friend Dr. Arthur Eowe, M.S., M.E.C.S., 

 F.G.S., of Margate. This gentleman's admirable researches on the 

 zones of the English Chalk have greatly added to our knowledge of 

 its detailed stratigraphy, whilst, by the application of the dental 

 engine for the development of minute and delicate organisms 

 preserved in the Chalk, he has made geologists acquainted with 

 a host of beautiful and novel organisms, among which the present 

 addition to our knowledge of the form hitherto known as ' Pyrgoma 

 cretacea ' is not, as I hope to be able to show in the sequel, the least 

 interesting and instructive contribution. 



Towards the close of last year. Dr. Eowe brought me the specimen 

 which is the subject of the present communication, and which is figui'ed 

 (enlarged three times) on PI. VIII, Fig. 4a. The original specimen 

 obtained from the Chalk of Norwich, and described by me in 1865 

 (see PL VIII, Fig. 3), consists of nearly half the circumference of 

 the conical walls of the shell, the opercular valves and the basis 

 being absent. 



I attributed the absence in the Norwich specimen of the character- 

 istic cup-formed basis, usually seen in Pyrgoma anglicum and other 

 species of that genus, to the readiness with which the conical walls 

 of the shell separate from the basis, owing to a cleft covered by 

 a membrane which may be observed all round between the lower 

 edge of the shell and the basis in many of the species. In referring 

 this Cretaceous Balanid to Pyrgoma, I was influenced by the 

 following considerations, namely : (1) the steeply conical form of 

 the shell-wall (see PL VIII, Fig. 3) ; (2) the rounded approximate 



^ A Monograph of the Subclass Cirripedia, etc. : The Balanidre aud Verrucidse, 

 p. 3.55. Eay Society, 1854. 



